Air-Source Heat Pump for DHW and In-Floor Radiant Heat
Hello!
I am retrofitting a 500 sq. ft. studio (ADU) in the San Francisco Bay Area. The building has radiant heating in the concrete slab. It currently has a gas boiler and a Taco X block to heat the radiant loop. I want to redesign the system with three goals:
1) More energy efficient and doesn’t use gas
2) Does a better job of prioritizing hot water for showers when the radiant heating is on
3) Uses less indoor space
My thinking is to take out the boiler and replace it with an outdoor monobloc ATWHP to serve both DHW and radiant heating. However, I’d like to reclaim the space that the boiler currently occupies. Are there any ATWHP designs that either a) don’t require an additional storage tank for DHW b) can work with a modest sized outdoor tank or c) some other solution all together?
Thanks all for the help!
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Replies
That’s a tricky one. Plenty of people in the South place water heating tanks outdoors (often in tiny lockers looking enclosures), that might be your best bet. You will not find a “tankless” air to water heat pump, their capacity is too low.
I’d definitely price out a simple ductless minisplit and consider ditching the in-floor.
The only way to go tankless and all-electric is with an electric on-demand water heater, and those aren't efficient (compared to a heat pump) and need hefty wiring. You're not going to find anything analogous to an indirect coil on a traditional boiler with a heat pump.
So for all-electric your choices are on-demand -- compact but inefficient -- or heat pump water heater -- efficient but needs space for a tank.
In terms of heating, you can't generally drop an air-to-water heat pump into a system that was engineered for a gas boiler. Boilers produce water in the 180F range, and heat pumps produce it in the 110F range. Sometimes boiler water is mixed with a tempering valve to make it cooler for radiant floors, that's something to look into.
I think your best bet from an efficiency standpoint would be a minisplit and a heat pump water heater.
Slab heating shouldn’t be using temps beyond a heat pump’s ability. But a ductless unit would be probably five figures cheaper.
Are tankless water heaters inefficient? I removed the 30gal tank unit from my mobile home and replaced it with an ECOsmart 18 about five years ago and aside from the joy over never running out of hot water, my electric bill definitely dropped a noticeable amount. I will admit the cost is about twice as much as what is on the box/website because as you mentioned the wiring and electrician cost is steep, but I would think my unit paid for itself in just a couple of years. The house I'm building now is 100% going to have an ECOsmart 27 in it
I did find this “tankless” heat pump:
https://www.intellihot.com/electron-ie1/
They seem to integrate a glycol tank with a co2 heat pump and offer instant hot water. But it seems loud and expensive. Can I build something like this myself with an Arctic or Macon heat pump?
Ha fair enough. It doesn’t have a tank of hot water, just a store of thermal energy in some non-water form. I’ll rephrase it as: there are no tankless or boxless air to water heat pumps. You could definitely use an air to water heat pump to heat something other than water if you want to avoid having a tank of water outside, then incorporate a heat exchange inside to act as a tankless.
I think that’d be overkill for a small place. Are the domestic hot water demands especially high? Fitting a small resistance tank (or devoting a lot of amps for a tankless heater) are both much easier. Or just heat the water outdoors and accept a freeze risk. It really depends if you want to spend $2k or $20k+
So seems like I need to compromise a bit. Here are the options I see:
1) ATWHP for just the slab radiant heating + a tankless water heater (electric with a circuit upgrade or gas).
+ fairly energy efficient
+ can be all outside
- requires two systems
- cost?
Q: I assume I can use an ATWHP without a tank here because the slab would have enough thermal inertia to prevent short cycling and the discharge temp doesn't matter too much. Is that right?
2) Ductless minisplit unit for space conditioning + a tankless water heater.
+ fairly energy efficient
- requires two systems
- cost? Maybe a little cheaper than option 1 because ATWHPs cost more than minisplits?
3) Give up the indoor space for a tank and do a combined DHW + radiant heating with ATWHP.
+ Most energy efficient option
- Complexity, cost
- Space requirement
4) Gas combi-boiler
+ Simple and space saving
- Not very energy efficient
Q: Do they make outdoor versions of these?
Do these tradeoffs sound right to you? Any other considerations you'd recommend I add in?
Thank you!
ATW heat pumps are extremely rare in the US. A ductless minisplit might cost $5k installed. I’d expect the ATW to be $15k+ easily - they have a lot of parts and labor that ductless air to air just do not.
“ I assume I can use an ATWHP without a tank here because the slab would have enough thermal inertia to prevent short cycling and the discharge temp doesn't matter too much. Is that right?” correct !
If you're willing to countenance gas, the on-demand water heater is where gas really outshines electric. I would consider a minisplit with a gas on-demand water heater, that's overall probably your lowest cost to install, lowest cost to run and least indoor space option.
"Q: I assume I can use an ATWHP without a tank here because the slab would have enough thermal inertia to prevent short cycling and the discharge temp doesn't matter too much. Is that right?"
The air-to-water heat pump I am most familiar with is Chiltrix. From the Chiltrix website:
" buffer tanks are needed whenever a Chiltrix heat pump interfaces with a radiant or floor loop."
https://www.chiltrix.com/small-buffer-tanks/
I want to expand on the reason for this a bit.
The "secret sauce " of minisplits is that they modulate -- the output can be varied at very fine gradation to match the load. You would need some sort of sensor inside the house to transmit load information to the compressor. I'm not sure if such a thing exists. In the Chiltrix system the buffer tank acts as a proxy for such a sensor -- a thermostat and pump circulate water from the tank to meet the heating load, and the compressor senses the water temperature from the tank to modulate the compressor output.
Note that the tank the use isn't very big, only 17 gallons.
Quick rough comparison of costs: you can buy a 12K mini-split for $600 or so, although you might want to go to $1100 or so for something a little nicer. A Chiltrix air to water heat pump plus buffer tank starts at $6100 for just those two pieces.
While I realize natgas is on the outs these days it is pretty hard to beat a condensing gas boiler for efficiency. They have wall mount units that take up little space, although as with most other solution, you will need a storage tank of some sort for hot water.
There is no reason to suffer hot water shortage while the heat runs. There should be a setting for hot water priority on your boiler that will shut off the circulator while there is demand for hot water.
While it might seem a shame to eliminate the radiant slab, as mentioned a mini split is soooo cheap that between that and a standard tank HPWH or other solution , and a pair of slippers, the radiant might have to go.
Depending on your hot water usage, a shorty water heater[do they make those in HP?] can be stuffed up above somewhere to clear up floor space.
Let's look at the overall picture. According to the weather stats I could gather, SF has a year-round average temperature of 58F, heating design temperature of 40F, cooling design temperature of 79F. Average of 2900 degree-days of heating (I was surprised it was that high).
A 500 SF slab could probably produce at most 15,000 BTU/hr, more likely it does more like 10K. Let's assume it's properly sized to do 10K at 40F outside. That's 333 BTU per degree-hour, so with 2900 degree days per year that's 23 million BTU per year for heating. That averages out to 2600 BTU/hr.
For water heating, let's assume 25 gallons per person per day, two people. Let's assume an average 40F lift, water enters at the mean annual temperature of 58F and is used at 98F. That's 8300 BTU/day, or 3 million BTU per year for water. Average load is 345 BTU/hr over the year.
The big issue with water heating is instantaneous demand and the shower is the biggest consumer there. Let's assume you need 2.5 gallons per minute for your shower. That's 830 BTU per minute or 50,000 BTU per hour.
Since the hot water and the heating have such different demand profiles it's unlikely that you're going to be able to find a single appliance that serves both. The annual usage for heating is about eight times that for hot water. However, the peak usage for hot water is five times that for heating.
Combined, you need 60,000 BTU/hr of peak capacity with an average load of 3000 BTU/hr. This is why tanks are popular! I'm going to go out on a limb and say that no heat pump can provide that without some sort of buffering. The heating load alone -- 10K peak, 2300 average load -- is well within the range of heat pumps. The problem is really the water heating.
Let me add that if your average heating load over the year is 2600 BTU/hr, with a 500 SF slab that's 5.2 BTU/SF. With an interior temperature of 72F that corresponds to a slab temperature of 74.6, which is going to be imperceptible.
Really the only options for a tankless water heater are gas and resistance electric. The drawbacks with resistance electric are that is roughly 3x less efficient than a heat pump, and that you need to have the electrical capacity.
For 50,000 BTU/hr you need 14.6 kW which is over 60 Amps at 240V, you'd need an 80 Amp circuit which is problematic in a residential setting.
The biggest thing you can do is reduce the flow rate in your shower. I've got to High Sierra showerheads, which give a very satisfactory shower at 1.5 GPM. That gets you to 30K BTU/hr or a 50A circuit. That's still a lot but it's more doable.
The annual energy consumption for a resistance water heater would be 880 kWh. I don't know what your rate is, at $0.20/kWh that would be $176. A heat pump would probably save half to two thirds of that, or $88 to $117 a year.
You could probably power that water heater with about 800 Watts of solar panels. I'm seeing installed costs of around $2/Watt after tax credits, so $1600. It only works if your utility offers net metering, otherwise you keep running into the same peak capacity problem.
Thanks for the calcs, @DC_Contrarian. We use the studio as a guest house and occasional office, so the occupancy is actually even lower. My conclusion is that the utility bill difference is small here - the real concern is install costs.
I'll have to see how much it would cost to upgrade the electrical for a resistance water heater, but my guess is not cheap.
Guest house? Minisplit and on-demand gas water heater and call it good.
I've seen exactly this (ducted heat pump + tankless) in some new condos. When space is at a premium, you can't beat a tankless water heater.
This also gets you cooling for free which is expensive to implement with air to water.
Thank you, all!
I guess one last option I can consider is just replacing the current gas boiler with a hybrid electric heat pump boiler and keeping the Taco X block heat exchanger for the secondary radiant heating loop.
Any concerns with that design? As far as I understand the Taco just circulates the hot water out of the tank to the cold water supply in the primary loop to take heat from the system. So the main concern would be the tank getting cold when the slab is heating up, especially after being off. Not sure there is a way to ensure DHW priority here.
I appreciate everyone's advice! I'm leaning minisplit + exterior on-demand gas water heater per @DC_Contrarian's suggestions. It seems like the most cost efficient and space efficient solution. Bummer to not use the radiant slab, but the AC is a plus.
Any recommendations for the best brands/models of minisplits to use? My goals are reliability, low noise, and the right capacity for the space. The building is fairly new (~15 years old) so the insulation is good, but it does have a vaulted ceiling which might impact heating/cooling.
In your warm climate, it doesn't really matter. For 500sqft, probably this is in the ballpark but it might be worth it to run a quick man J on it (ie coolcalc or hvac.betterbuiltnw.com ):
https://ashp.neep.org/#!/product/34425/7/25000///0
At the end of the day it mostly comes down to the equipment your installer is familiar with, but I would still check the neep site to look at the modulation range of the equipment. Some wall mounts don't have good turndown at higher outdoor temperature and will be cycling a lot in your mild climate.
The bed/bath will need some aux heat. You can use the existing floor heat and add a small resistance boiler or keep the x-block and run the bath/bed floor heat off the tankless. A couple of plug in heater or small baseboard would also work.
Thanks, @akos! I'll run a coolcalc on it to ensure correct sizing.
I like the idea of reusing the x-block with the tankless water heater for supplemental floor heating. Can I basically follow this schematic (but with just 1 zone in my case):
https://www.radiantcompany.com/wp-content/uploads/heatexchangeschem.jpg
Will most tankless water heaters work in this application or what should I look for?
This article covers the subject:
https://www.greenbuildingadvisor.com/article/using-a-tankless-water-heater-for-space-heat
Thanks for the link!
Seems like the main concerns the article raises are:
1) stagnant water in the heating coil or heat-distribution coil
- My system would be "open indirect" so the stagnant water is minimal: just between water heater and heat exchanger.
2) return water temp too high
3) need high supply temp for hydro-air
- Doesn't apply because it's a radiant slab
4) Need priority control and cycling control
- Again doesn't seem to apply because of the "open indirect" design. Am I wrong?
5) Short cycling
6) Cold slugs
My conclusion is the main goal is selecting a tankless water heater with a large turndown ratio and setting the Taco X-block to a low flow rate in the primary loop to minimize the return water temp.
What am I missing?
In your existing system, does the heated side of the X-block take boiler water or domestic hot water?
If it's ever been used for anything other than domestic hot water you can't attach it to potable water.
The heated side of x-block has always been used with DHW. It previously connected to a dhw hot water tank (standard gas tank water heater).
OK, thanks. I was a little confused because in the initial description you said you currently had a boiler that was feeding the X-block. But it's a tank water heater?
And that's giving unsatisfactory performance now, there isn't always hot water when it's wanted? The reason I ask is that one of the things Martin talks about in the article is that the demand profiles for space heating and hot water are so different that it's hard to get a single device that can meet both. Using a tank is the time-honored way of smoothing out demand. If your system isn't working well now with a tank water heater I can't imagine it's going to work better with a tankless.
In many locations code prohibits mixing domestic hot water with the heating water. It is also almost impossible retro fit the plumbing in an existing building properly for a mixed system.
If you get it wrong you could kill people to save what a hundred dollars a year maybe.
Walta
@walta. That’s not the case here: I am considering using the taco x-block to isolate a radiant heating loop from my dhw loop with a heat exchanger. The DHW loop would be heated by a tankless water heater as in this schematic: https://www.radiantcompany.com/wp-content/uploads/heatexchangeschem.jpg
Any other issues I’m missing here?
That schematic shows a standard hot water heater.
I do not know enough to say that it won't work with a tankless, but when using something in a way it was not intended, sometimes the project never ends.
heatinghelp.com, lots of hydronic guys who will know for certain
I've used both Rannai and Takagi condensing units for this without issues. Make sure to check that the unit you choose is rated for combi heat (most are). Using it for combi heat reduces the warranty in most cases.
Neither unit had issues with return water temperature but if the heat load is bellow the unit's min fire they will cycle which can wear them out. I haven't use the X-block, can't help you much there, but I would guess you'll have to do a bit of tweaking. Adjusting the flow to get higher delta T (~30F) on the water heater side is a good start.
Make sure to watch the fresh water runs to the X block as most codes limit this to 50'. Even better, put the x-block right by the heater. Adding a timer or setting up the system so that cold water is always drawn through the XBlock is also a good idea to avoid stagnant water issues.
There is also no efficiency lost by running the tankless hotter, best is always to run them at 140F and mix down to 120F for DHW.