Electric boiler for both DHW and radiant heat
I am trying to figure out if an electric boiler with heat exchanger can handle both domestic hot water needs and radiant infloor heating.
The assumptions I am using is heat load of 58,000 BTU over 2,400 square feet, with hot water required in 3 bathrooms by children who like their baths and showers long and hot (family of 5).
The options on the table are Combomax 50 (seems entirely insufficient for DHW needs), or some kind of Thermolec or Electric Industry boiler plus het exchanger plus tanks. E.g. 20Kw models capable of 60+ BTU.
(1) Any tool I can use to evaluate the size requirement?
(2) Any especially good options for hot water tanks (there seems to be no? indirect water heaters for a small scale)
(3) Should I just give up and keep it simple with a 9Kw boiler/water heater and 11 Kw boiler for hydronic floors?
There are all suggestions that mechanical engineers proposed, though the numbers are odd to me.
Heating dominating climate, zone 5A.
Thoughts? Questions I should be asking my mechanical engineers?
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You could use an indirect + boiler. You can find indirects in the 30-80 gallon range. I think two separate systems would be easiest though. I hope your cost per kWh is low!
58,000 btu is 17kw, so don't go the 11kw boiler route for hydronic.
What are you trying to achieve, on demand hot water? If so, at 2.5gpm (typical shower head) you'll want another 18kw at a minimum. If you want to provide for more than one faucet, you'll need a bigger one yet. With that said, electric heaters are so cheap, I wouldn't combine DHW and Radiant when going with just resistance heat. I would use a standard water heater, or two in series if you need more run time. This would get you to 9kw on the DHW side too. And then a simple tankless heater (rated for heating) for the radiant. You'll spend more in plumbing trying to combine them than the cost of the equipment to just keep them separate.
Just a note - if you decide to go the route of a hybrid heat pump water heater, do not run it in hybrid mode during the heating season. Since your house heat is provided by resistance, this will result in an overall COP of 1 with extra wear on the heat pump that isn't necessary.
Without knowing what you're trying to achieve it's impossible to answer. This is not a common configuration, for the reason that in most situations there's a better configuration.
I do not have gas line and I need some energy source for the domestic hot water and hydronic heat.
The plan was always to install electric for both.
I am limited to 20Kw because I have 200 amp service only and various "back of the envelope" calculations indicate it is 20Kw for both.
Hence the idea of combining the two so that one can override the heat to supply hot water for the 30 minutes people take concurrent showers.
I see no other way around it.
The 11Kw source (vitotron boiler) with 30 gallons storage tank to supply in floor hear puzzled me immensely but it came from the person who designed the mechanical system originally. I think the idea is that under the hardwood radiant needs a lower temp and the storage tank will permit mixing hotter water from the boiler to get to the right supply temp for the hydronic but to me a BTU is a BTU....
Hence the quest for insulated tanks - how much water can I store playing on the diff between the boiler supply at 180 and mixing down for the infloor (115) and domestic (whatever is the best point).
I have two mechanical engineers neither of whom WANTS to think about this because they do not seem to like hydronic or electric boilers.
Go with an air-to-air heat pump and a tank water heater, either heat pump or regular resistance.
There was just a thread a couple of days ago from a guy in Whitehorse, Yukon. I thought for sure there was no way a heat pump would pencil out in his climate but when we ran the numbers it made sense.
I wouldn't recommend hydronics, a lot of cost and complexity for not a lot of benefit. And definitely no efficiency benefit.
Okay given that your heat loss is 20kw and you have 20kw of capacity, you need a 20kw boiler. Full stop. I get the “undersize and use storage to get through the coldest hours” but there’s no real point if you can fit the 20kw one. Electric boiler capacity is cheap. If you can’t do a 20kw, get as close as possible and then consider a buffer tank, but a 20kw boiler will be much cheaper than a 15kw + storage.
From there, you need an indirect. There’s little downside to installing a large one, so say 80 gallons. That should be all!
What do you mean “20kw for both”? DHW is not 20kw alone.
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Thank you both. The choice of the heating is not on the table. The only choice at this moment is what heat source to use to heat the water for 2 applications
(1) Domestic hot water
(2) In floor heat.
What I am trying to ascertain is that an electric boiler with an indirect water heater can handle BOTH. I have 20Kw to spare
Thermolec models are here - Standard 20Kw gives 68 BTUh max, 3phase 18Kwh gives 61 Btuh.
http://www.thermolec.com/_documentationcache/cd-264-English%20Boiler%20flyer%20booklet.pdf
Electro Industry boiler - 20Kw gives 68BtuH
https://electromn.com/wp-content/uploads/2021/03/BL603.pdf (they have other models - I do not even know which one is better - eg https://electromn.com/wp-content/uploads/2020/04/BL509.pdf)
Combomax 50 does not seem to generate enough hot water based on the literature (5 min shower, 2.5G, etc) and I do not have neough Kw for Combomax 70.
https://www.thermo2000.com/en/products/combomax-ultra/
I cannot find and contractors are handwaiving the indirect hot water equipment.
The answer is yes, a 20 kw boiler + indirect can handle both. The way it'll work is that the DHW will have priority. If the indirect is generously sized, you'll have enough hot water for 5 people with room to spare. To put it in context, a typical electric tank water heater will have a 4.5 - 5kw element (2 but only one works at a time) and 40-80 gallons of storage. You could install a similarly large tank, say 80 gallons and you'd have a 20kw boiler attached, so capacity would be significantly more than what the typical house uses. Your first hour capacity would be about 3x a typical electric tank heater of 55 gallons.
There are plenty of indirect tank manufacturers: Lochnivar , HTP and US boiler are a few to start with.
"The choice of the heating is not on the table. "
"I cannot find and contractors are handwaiving the indirect hot water equipment."
So basically you designed a system in your imagination and now you're finding that it doesn't really exist.
If you're determined to go with resistance heat, baseboards are simpler, cheaper and easier to control than anything hydronic.
I know you said that you can't do heat pumps because of the noise, but I think you should revisit that.
The reason what you're looking for is hard to find is that it's rarely a good choice.
I did come here asking about acoustic heat pump covers. There seems to be none. There also no high-BTU heat pumps with under 50DbA sound pressure (not measured at 10ft distance which does not pass for me).
I can try importing a heat pump acoustic cover from Europe but that (1) is absurd, (2) may not pas muster (3) the only ones I found are not for the heatpumps I can get in North Ameirca.
Believe me, I have gone quite far down this path and have already looked at Chiltrix, Arctic, and Nordic. I even asked about them here. At length.
And due to an urban location split design heat pump has been ruled out due to dBa and noise limits.
Your contractor that is proposing an 11kw unit for an 18kw heat load because radiant floor temps are lower... you are right, a btu is a btu.
So your limitation is critical here and my first answer is void because of it. You have 20kw to dole out between your DHW and your Envelope heat load. You will not be able to heat your home at the design conditions while creating much hot water.
In this scenario, if your only option forward is resistance electric for hydronic and DHW, then a single 20kw heater is what you need. It doesn't matter which one, back to the original point, 20kw is 20kw is 68,240BTUs. Given that you know you need a large water supply, I would have a large storage tank. You can use normal water heaters with the elements unplugged or specific indirect ones, it doesn't really matter. It's just an insulated tank.
The combomax has a 20kw version that you can get in both the 50 and 70 gallon tanks.
Thank you.
The Combomax 70 has these assumptions on the hot water:
Domestic hot water evaluation based on:
15 minutes peak period
Based on 2.5 gpm shower head and shower duration of 5 minutes
Filling bathtub of the first bathroom
When 2 or 3 bathrooms are used, the second and third bathroom is a shower load.
Dishwasher load not simultaneous with the bathroom(s) load
Based on domestic cold water temperature at 40F
Based on water boiler temperature at 180F
Domestic hot water priority in function, included with the equipment
Based on use of mixing valve included with the equipment
Building heating load based on:
Table surface heating values refer to the total surface heated by the COMBOMAX ULTRA 12
The heating is based on an outside temperature of about -22°F (-30°C).
These values are for buildings built after 1990
I
https://e4m6s3b8.rocketcdn.me/wp-content/uploads/produits/ComboMaxUltra/combomaxULTRA-guide-EN.pdf
Their outside temperatures are lower than I ever need - outdoor design temperature is 0F, with indoor design temperature of 71.6F. But the 70Gal tank still seems small which is why I thought a Thermolec or Electro Industries paired up with the larger tank would be better?
I think a bigger tank would be helpful in your scenario. It sounds like you need a pretty good amount of hot water supply. Don't look to much into their generic specs. If you already know your heat load, they don't really apply. They are guesstimating a heat load for a generic house. If going with the combomax, you'd get the 20 kw model with 70 gallon tank.
Don't use that link to size anything. You know your heat loss. From there, just add an indirect. The bigger the indirect, the more water you'll have in a peak hour.
To add perspective. If your heat load is 18kw (58,000 btu), and you have a 20kw heater, you have 2kw left over to heat water during those times. 2kw will take around 4 hours to reheat a 50 gallon water heater to 120*F. 50 gallons allows you to take a ~20minute shower.
Precisely. But Electro Industry or Thermolec or even Combomax allegedly can be made to prioritize DHW, so even if the temp drops for 30 min for the floors.
Hence thinking 2 tanks - one for floors, one for DHW?
If you've got the room for the tanks, it might be worthwhile to have both. Controls like that would be useful.
What tanks should I ask for?
I don't think that's a good use of money. The max heat load occurs only a few hours per year. You'd have to time peak DHW usage with the coldest hours of the year which occur often at like 4am. The worst that would happen is that the boiler would spend under 1 hour ((80 Gallons x 8.34 lbs/gallon x 90 degree F delta T) / 68000 btu) focused on recovering the indirect tank, during which the house might have a slight temperature decrease. You'd probably never notice.
Maybe just live with 1 tank for a winter and decide if it's a problem worth spending on.
If the heat load calculations are correct, you might notice. 58,000 btu in a 2400 sqft home over an hour is pretty significant.
And it was not a contractor. That was a mechanical engineer who originally calculated a heat load of much higher BTU (over 70,000 heat load) which made me balk (for a 2,400 square feet house in 5A). With the help here (and two other mechanical engineers) we pushed it down to may be 58,000 or somewhere between 62,000 and 48,000 (they differed with each other too).
The 11Kw boiler was proposed by the oversized (over 70,000 BTU)engineer who said his choice of outdoor design temperatures (much lower than I need) is a "feature, not a bug". I got very confused at that point how 11Kw boiler is going to supply even 58 Btu.... I still do not get it.
They were wrong and you should get a refund.
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Are you certain on the 58,000 heat load? That seems a bit high on a modern build, even that hits code minimum.
My parents have a 2800sqft 2 story home on the border of 5/6 climate zone. Built in the 90s, 2x6, 24" on center. Maybe R25-R30 in the attic? Quite a few wood frame windows. Based on their propane bills, my dad and I recently estimated between 35,000-40,000 BTUs. I would say it's not a particularly air tight home.
No, I am not certain. THe more I looked into calculations (run one myself, then looked for differences among 3 calculations) - there are so many assumptions there. Which is why I am wondering if I should go with the simplest one and then add another tank later.
Unless it's a heat pump, there's really no penalty to oversizing heating systems. If you design to the 99% design temperature, there will be on average 88 hours a year below that temperature, and your system won't keep up and comfort will suffer, at least a little. If you instead design to the coldest temperature ever recorded you're pretty much guaranteed to always have enough heat. That's what he was doing.
That was for a heat pump.
If I go with COmbomax 70 (20Kw) - and find myself short of hot water (domestic) - can I then add a second tank?
I am thinking everyone has really been guessing so far. The estimate for heat load is all "assumption-driven".
you could also do an interlock. If you have several stages for the heating you could block the last ones if the hot water kicks in - basically the some power supply switched over either to space or water heating. Install the 20 kW in total, the space gets what is not used in the hot water. Ideally this function is already implemented in your control.
Basically, the weak points of an all-electric house is the energy source. Heat pumps available in North America do not provide the abundant hot water supply that separates North America from Europe where tepid low flow is acceptable. The noise they produce does not pass European standards - e.g. 48dbA at a property line - or the local standard for the unfortunate few of us who have noise ordnances in urban settings.
And while dbAs are not additive, a row of underpowered (weaker) heat pumps (with slightly lower dbA) which make marginally less noise is also a design nightmare.
But nobody this side of the ocean seem worried about it.
If you're using radiant floors, high temps aren't necessary. But otherwise, they're loud yes.
You can use a tank to overcome low flow.
That is what I am trying to figure out. How big. And how to heat the tank.
You only have one option here if you have only 20 kw to cover both DHW and heating a radiant floor and have a heat load of 20kw-ish: an electric boiler with indirect.
Well, they brought me multiple options - hence my questions here.
You ask 3 people, you get 6 opinions.
The most logical to me is a single boiler with a heat exchanger and two tanks (or the combomax which is pretty much that) but I may be entirely misunderstanding the issue - hence my questions.
In terms of sizing hot water, agree that it's largely assumption based. Are you living in a house now with the same cast of characters? The best way is to measure your current actual usage and use that.
We are currently hitting the limit on the hot water (not enough for a bath and two showers) but the question relates to a newly being built house.
So I have to schedule and stagger everyone's baths and showers and it is exhausting to mediate (I am a working mother, so evening hours are rushed, mornings are rushed and then when they want to either shower or take a bath all at the same time - because similar school starts and similar bedtimes).
What's the current water heater? tank (what size) or tankless, and how much input power?
DC is asking what you use now because your water usage schedule will likely not change in your new home. Are you running a 40gal tank on electric or gas now? Knowing the current usage and constraints can help you design for the new one so you don't have the same issues. It sounds like you need a larger than average water supply on top of an 18kw space heater. You may need to look at either increasing your panel/service size, or if the building is still in the design stage, finding ways to reduce your heat load. Based on our above conversation, I do think it will work, but there will be limitations to the system in general. If you can even fit a single 18k or 24k mini split head to assist with the resistance heat, it will take a large load off your water heater needs.
Not sure what electric costs by you, but a 58,000 BTU heat load would cost me (also climate zone 5) $6200/year in resistance electric costs. My rates are $0.17/kwh. I used the Heating Degree Days for last winter to calculate this.
I am not 100% certain as this is a rental house, but it looks like may be 50 gallon gas.
And I did the math on electricity rate - while the highest TOU rate at the moment is 15.1 I am hoping to average a little less than that (because current Tiered rates are 10c for highest tier)
https://www.oeb.ca/consumer-information-and-protection/electricity-rates?
and some solar.
Easy - indirects don't come in infinite sizes, but an ~80 or ~110 gallon option would easily increase capacity. You'd have both more stored energy and a more powerful heater attached.
So worth keeping in mind that 50 gallons isn't enough it sounds like. A typical gas water heater puts out more heat than a standard electric water heater. A typical gas water heater is probably around 40k btu, or ~12kw. If that's causing delays in a busy schedule, you won't be happy with less in the new house I'd guess.
As DC mentioned, you've unfortunately designed yourself into a bit of a mess here. 20kw will get the job done, but you might be making compromises. A simple sound barrier wall on a heat pump might be all you need to reflect the noise away from the property line. A wood fence would do it. In Europe, I have never seen a heat pump with an acoustic cover, and I have seen them right on the property line.
Actually, a wood fence would not deflect enough noise from the heat pump given the limit on wood fence heights, noise emitted, and sound travel.
Done the math using sound powers and all.
Given your climate zone and being Ontario, your weather is probably quite similar to here in Detroit. No one has A/C in your city?
Sorry, I know your originally said no heat pump. Given your constraints, electrical rates, etc. The biggest water tanks you can fit along with any 20kw electric boiler rated for space heat is what you need. Even with your low electric rates at night, your heatload is going to use a lot of power during the day. It's going to be pricey.
They do. Which is why we have noise limit bylaws because the sound from outside ACs drive people up the wall.
Just to clarify - you are building a house in a hot/humid summer environment with no A/C? And in this hot/humid environment, the city has made it basically illegal for anyone to install A/C? What city?
Just as an aside -- I have switched to High Sierra showerheads low flow showerheads and I highly recommend them. I have a family of five and a 65 gallon tank and everyone can take a shower in the morning. The flow is entirely satisfactory, no complaints.
Here's a link:
https://www.amazon.com/gp/product/B09VD22H8W
Given all your constraints reducing usage is looking like it might help a lot.
It may be part of the problem - we are installing decent but low flow shower heads (the showers in the rental is a bit Niagara-falls like).
Switching out a shower head is about a 2-minute task, just keep the old shower heads and switch them back when you move out. It will allow you to see if you find the low-flow head acceptable, and might just solve your hot water supply issue now.
Let's say your family uses 80 gallons of hot water a day. Let's also say that water is heated to 110F and enters at 50F, a 60-degree rise. So that's 80*8.3=664 pounds of water times 60 degrees= just under 40,000 BTU per day, or 11.4 kWh.
If you have an 80 gallon tank, you can get that 11.4kWh by using 11.4kW for one hour, or 0.5kW for 24 hours, or really anything in between. For a regular tank water heater you can get replacement elements as low as 1kW, and you can get them to draw as little as 0.25kW by running them on 120V instead of 240V.
If your heating load is 58K BTU/hr that's 16.5 kW. So the simplest thing to do to stay under 20kW is to get 16.5kW of resistive heat and get a big water heater tank and replace the elements with elements rated under 3.5kW. Or even two 50 gallon tanks in series, you can wire them so only one runs at a time. With a hundred gallons of capacity and 3.5kW elements you're looking at 4 hours to completely replenish your hot water supply, which means you could easily use 100 gallons in the morning and 100 gallons in the evening.
I have to reiterate that if you're using resistance heat there is absolutely no benefit to using hydronics. If you have to have under-floor heat you can run resistive heat under the floor. It costs the same to operate and the installation cost is probably one tenth. Electric baseboards would also have the same operating cost and would probably be a quarter of the installation cost of underfloor electric.
Have you walked around the neighborhood to see what your neighbors do for air conditioning?
I think a benefit is the future option to switch to a heat pump, which would be extremely efficient in a low temp application like this.
I started wanting a heat pump and went very long way looking at Chiltrix/Arctic/Nordic until running into the sound level problem.
When either the heat pump acoustic covers become mainstream here or quieter heat pumps more available here, I would happily go back to the same idea.
So I was just reading this thread:
https://www.greenbuildingadvisor.com/question/return-air-grille-vs-duct-distance
and realized you are the same author. Is this the same house? You have ductwork but can't have a heat pump?
Is this your sound code? You're in a city, climate zone 5, Ontario... Making an assumption.
Mitsubishi heat pumps are rated up to 51 db. But keep in mind, the below code is 50db(A) which is a weighted scale. A very critical distinction. In other sections of the code, they reference 50db(A) OR 65 db(C). I find it hard to believe that a city in this climate zone where it's hot and muggy, is outlawing A/C. I don't believe that's what this municipal code is doing though.
https://www.toronto.ca/legdocs/municode/1184_591.pdf
"§ 591-2.8. Stationary sources and residential air conditioners.
A. No person shall cause or permit the emission of sound from a stationary source or
residential air conditioner that, when measured with a sound level meter a point of
reception, has a sound level (expressed in terms of Leq for a one-hour period) exceeding
50 dB(A) or the applicable sound level limit prescribed in provincial noise pollution
control guidelines.
B. Subsection A does not apply to the emission of sound from a stationary source that is in
compliance with a provincial environmental compliance approval."
LEQ is the weighted average sound level over a time period, in this case an hour. So unless you have 100% duty cycle the LEQ is going to be a lot lower than the instantaneous level.
LEQ was deliberately created as a bogus measure to make it seem like you're doing something about noise.
Mitsubishi heat pumps sound measurements are taken 1 m from the pump (3 feet or so). Other heatpumps report measurements at 10 feet or 10 meter or whatever makes that number look low.
I already have a mitsubishi for the AC - sized for a much lower cooling load - which is what the ducts are for - and capable of adding some heat.
And it may or may not squeak by our "noise police" (there is a separate city staff that measure noise created after the bylaw was created in 2019 who stick with 50dBa limits) but I think with a homemade acoustic cover (which is non-trivial given one cannot block air access per manufacturer) I might just be able to pull it off.
The plan was to have a second heat pump to heat the water for the hydronic heat and DHW until it turned out that ATWHP (Chiltrix, Arctic, Nordic) are much much louder when you drill down to how they measure and report the sound. The noise from two heat pumps is not purely additive, but even ALONE there was no sound barrier I can add to bring it down to 50dbA at the property line. Sanden does not provide enough capacity - quieter as it is.
And to reiterate - I am on an urban lot, with setbacks and little room to play with to begin with.
Please, I did start wanting a heat pump. I did not abandon it easily.
How much heat can the Mitsubishi add?
I think you'll be miles ahead if you get as much heat out of that heat pump and supplement with floor heat strategically.
If you're heating with a heat pump it then starts to make sense to have a heat pump water heater. The heat pump water heater will give you free cooling and dehumidification in the summer.
So you already have a heat pump? I would make sure that's a cold climate one, and subtract that from your heat load. Now you can much more easily use your 20kw for DHW and radiant heat. You can really take advantage of your low night time rates by using the heated floors in the night, then switching over to the heat pump to carry you through the day when temps tend to be higher anyway.
Thank you. SO nice to be heard.
It is Mitsubishi cold climate one.
I am vascillating between what you describe - or just take Tiered rates (highest 10c, average would be around 9.50) and run hydronic steady heat all cold months supplementing with air heat pump as needed.
Do you have a model number for that heat pump? That will help significantly in this.
I'm suspecting that the mechanical contractor that suggested an 11kw water heater for hydronics, suggested that because your ducted heat pump is making up the other 6kw of your heat load, if not more. In that case... I'm back to the beginning, I wouldn't necessarily combine DHW and Hydronics unless you really want to spend the money on the Combomax. An 11kw boiler for hydronics and two 50 gal 4.5kw electric water tanks in line would be a great setup, and save $3000 from the combomax. Use a tempering valve, heat them to their max rated temperature at 3am on your ULO rate and you'll have enough water to get through the day without triggering your higher electric costs for DHW. On the radiant side, I'd just set it back a few degrees during your peak hours and let the heat pump work hard.
In response to #58 - agreed, a boiler + indirect is the best option and preserves the future possibility of a heat pump. The combomax is a great option since so many components are included in one package. One of the best things about electric boilers vs. gas/heat pumps is how low the head loss is, so pumping energy use should be low. I like how it offers the option to decide when to cut off central heating while DHW is being used. Often that’s a just a binary. If you can maximize the ultra off peak use, you’ll have a fine system.
I think the second buffer tank is overkill to start, but you can always keep the option open.
My knee-jerk reaction upon first encountering this thread was "an electric boiler is never the solution." Nothing I've read has changed that opinion.
Ha fair enough. I think if all of these things are true, it's a good option. I wouldn't consider one in 99% of situations.
1. In-floor heating is preferred
2. Limited amps available
3. Heat pumps not available/allowed currently
4. Heat pumps might be available/allowed in the future
5. Electric rates are favorable
6. No gas available
They're not terribly expensive and can be abandoned/set as backup easily enough.
If 1 and 4 were different, I think electric baseboard probably makes the most sense.
The Arctic Heat Pump website has some articles in which they talk about being able to hook the heat pump up to solar thermal, the buffer tanks you should use, and how to heat DHW (which can also be helped with the solar thermal).
Are any of these options for you?
https://www.arcticheatpumps.com/heat-pump-with-solar.html
https://www.arcticheatpumps.com/buffer-tank-for-heat-pumps.html
https://www.arcticheatpumps.com/heat-pump-water-heaters.html
Eco-Ultra Buffer Tank
https://www.arcticheatpumps.com/eco-ultra-heat-pump-buffer-tanks.html
Orange_cat has rejected that over noise concerns.