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Upgrading HVAC Mechanical Systems

nateflanigan | Posted in Mechanicals on

Hello – I recently moved into a 1920’s craftsman in climate zone 4a. The house is very leaky and has a pretty old boiler heating system. I’m eligible for some helpful rebates and financing options through our utility company and I have a somewhat sensible budget to work with from the sale of our previous house. I feel that I have a good grasp (and reasonable expectations) on what has to happen in terms of air sealing and insulation but I’m still very much in the weeds on how to approach the mechanical system.

What I have:
– I did a blower door test with an experienced friend yesterday; 5500 @ 50 with the basement door open 4500 without. Square footage is about 2800, I’m guesstimating the envelope surface area at 6,590 sq ft and volume at 43,560 cu ft. Seems to come out to 8 ach.

– Gas fired atmospheric vented hwh from 2001 (poorly vented)
– 140,000 but Boiler from the 90’s
– Cast iron radiators in parts of the house, but most of the main living areas are hydronic baseboard heaters.
– No air conditioning
– Solar array that produces on average 4200 kWh per year

Proposed upgrades on the table:
– I have a good proposal for air sealing and increasing insulation in the usual places. Based on the wisdom available on this site I’m assuming these efforts could result in a 20-30% improvement in air leakage. There are A LOT of big holes to fix (which is good in a way) but I do realize I’m very unlikely to ever get to something like 3 ACH with the budget I have.

– Replacing the hwh and boiler with a Bosch combi unit – $13000 https://www.supplyhouse.com/Buderus-ZWB-42-3-Greenstar-151-117000-BTU-Combi-Gas-Fired-Wall-Hung-Condensing-Boiler

– Ducted heat pump HVAC system. The details of this are still being worked out but I’ve received one proposal for a 5 ton system with two air handlers for $35000

What I’m hung up on:
– The existing baseboard heaters heat the air though convection, which leaks out of the house pretty aggressively, currently if I set the thermostat in my living room to 70 it gets toasty, the thermostat clicks off and the room only holds the heat for about 20 minutes. Even improving this by 30% only gets me to 26 minutes. I could upgrade the baseboard units to panel radiators, I think I’d need about 10 units at ~ $500 a piece.
– As there’s no AC, I really hope to be putting in some sort of system that would  include a compressor, ductwork, electrical work etc. Given that it’s an existing old house a big central air handler and big ductwork system would be difficult. Which brings me to a zoned heat-pump system at which point I’ve also got heat. I’m all for belt and suspenders but paying for two premium heating systems seems foolish (maybe it’s not, I could be wrong, I often am)

So my questions are:
– does upgrading the boiler make sense if I’m buying a heat-pump system?
– given that the house came with a solar array does it make sense to move toward electric? Install a hybrid hwh and heat pump hvac and keep the old boiler for worst case scenario back up heat?

Thanks in advance for any advice

 

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Replies

  1. Paul Wiedefeld | | #1

    "Does upgrading the boiler make sense if I’m buying a heat-pump system?"

    Not particularly - the efficiency of heat pumps is higher and the conditions where they really shine (milder temps), they'd really outperform the boiler. The "high" efficiency boiler likely won't be much more efficient than the existing boiler at the lower outdoor temps. A tighter house will allow more efficient operation though, but that's true for both systems. Obviously, a new boiler doesn't provide AC, so it's $13k for not much benefit.

    If you can get last years data from the utility or are willing to wait a month or two, this method will help you determine just how big a system you need (5 tons might be too big): https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

    "Given that the house came with a solar array does it make sense to move toward electric? Install a hybrid hwh and heat pump hvac and keep the old boiler for worst case scenario back up heat?"

    Definitely - the hybrid approach is great, but depends on the gas meter fee and gas cooking preferences. Sometimes if the monthly fixed cost is high enough, it's worth ditching gas entirely if you're not relying on it for heat and would consider cooking with electricity. Consider the solar electricity nearly free: you didn't pay for the installation and probably didn't have to pay much of a premium for them AND that premium is financed with a mortgage.

    "The existing baseboard heaters heat the air though convection, which leaks out of the house pretty aggressively, currently if I set the thermostat in my living room to 70 it gets toasty, the thermostat clicks off and the room only holds the heat for about 20 minutes. Even improving this by 30% only gets me to 26 minutes. I could upgrade the baseboard units to panel radiators, I think I’d need about 10 units at ~ $500 a piece. "

    Believe it or not, but radiators don't radiate much heat either: they mostly use convection. You can see this most clearly with panel radiators: a 2in radiator puts out much less heat than a 4in one of the same dimensions. That's because it has more fins and allows more convection. You could try to lower temperatures in the baseboard to keep them at a more stable temp, but your boiler is most likely oversized by 2-3x so it'll take some effort to prevent it from short cycling even more than it already is.

  2. Cody Sibell | | #2

    Paul is spot on (though I slightly disagree with the blanket statement that radiators don't radiate much. This depends on the geometry of the radiator, its surface temperate, the temperature difference between the top and bottom of the fins, and the geometry of the fins. A tall, narrow radiator run at a high surface temperature with 2" fins will be proportionally more radiant than a short/long radiator run at low temp with 4" fins.) It's a difficult decision, with some tradeoffs on either side. Some more things to consider:

    How much of your current PV capacity are you using? If you're using all of it already, you won't have any "free" electricity to use for your heat pump system. What is the relative cost of gas and electricity in your area? Does your electric utility offer an electric heating rate (mine does, which cuts cost considerably but requires you heat exclusively with electricity)?

    Will you be able to eliminate drafts in your house sufficiently to make forced hot air heat comfortable? What is your wall construction and insulation? Many people find radiators much more comfortable than forced air in drafty houses, but can't tell a difference once sufficient air sealing occurs. It is nearly impossibly to make no-or-minimally insulated brick comfortable with forced air. What type of windows do you have, and how drafty are they? If they are single-pane (or even un-coated double-pane), you may have a significant reduction in comfort if you remove radiators/convectors that are currently under the windows and and are unable to replace them with forced-air registers.

    I think it likely that a "standard" ducted HVAC system would be a hugely invasive installation likely to compromise (if there are any) original and irreplaceable details in an old craftsman house. Duct sizing/design and register sizing/placement are critical for a successful (comfortable) installation, and hard to execute to requirements in an old=home retrofit. Would an air handler end up in unconditioned attic space?

    A mini-split would probably be better than a regular split. These systems can have small air handlers that can be creatively hidden so you don't have intrusive units on the walls, and can modulate both heating/cooling capacity and fan-speeds for long run-times. But the issue of having a sufficiently tight and insulated envelope still applies.

    A few specific replies to your questions: 1) Convective heat doesn't "leak out" at a higher rate than radiative heat. It is less comfortable in a drafty/poorly insulated house for a variety of reasons. 2) You can't really calculate that improving air sealing by 30% will mean your room will "hold heat" 30% longer before the thermostat clicks on. A 30% reduction in air leakage does not directly translate to a 30% reduction in BTU loss. But you do touch on an important point. You will find substantial comfort gains with a system that MODULATES its output so that it is nearly constantly heating/cooling based on the heat loss/gain of the space instead of old-fashioned on/off controls, which result in constant temperature swings, and lag-time. Oversized input (boiler/furnace) as well as convective heat source exacerbates this, particularly in a leaky house.

    Sorry if this has more questions and answers, but there is a lot to contemplate with a project like this. Old houses are challenging when it comes to space conditioning upgrades, and each project is unique. Installation costs/comfort/operating costs/carbon footprint concerns rarely leave an obvious perfect solution. For heating comfort in an old-house retrofit, forced air is not going to beat a properly-sized mod/con boiler with properly-sized homerun panel radiators with TRVs. In even an averagely leaky house, it won't come close. But, it doesn't give you air conditioning or an easy carbon-free future.

  3. William Hullsiek | | #3

    I would do a room by room manual J and try to balance out your emitters, so you get more even heat. Low temperature hydronics is very doable with a cold climate heat pump. Some heat pumps have built in electric boilers for low degree days.

    The Coffee with Caleffi series has several episodes on heat pumps, check out You Tube.

    When you do the math look at your cost per Therm or cost per KWh., to compare electric vs. natural gas. You may decide to pay more for electric in the short term.

  4. nateflanigan | | #4

    Thank you for the replies - I was attempting to balance providing enough information without detail overload in my original post.

    - How much of your current PV capacity are you using?
    I don't know. I've only been in the house about two weeks. PSE&G is $0.12/kwh and $0.32/therm.

    -What type of windows do you have, and how drafty are they?
    relatively modern double glazed, though somewhat leaky. It's hard for me to quantify that. I think touching up the weather stripping on some of the leakier windows would help. When we did the blower door test they didn't feel crazy leaky.

    - Will you be able to eliminate drafts in your house sufficiently to make forced hot air heat comfortable?
    I don't know. There is a lot of room for improvement but I'm trying to be realistic about how far I can expect to get. I met with an hvac estimator today who works with the insulation/energy auditor I'm using. Based on her experience with the energy guy she felt he could probably cut the blower door number in half. So, I'm cautiously optimistic.

    -What is your wall construction and insulation?
    The original part of the house is ballon framed 2x4 walls, no insulation that I can see. There's an addition built in the 90's that comprises the most used living spaces. 2x4 walls r13 batts.

    - Would an air handler end up in unconditioned attic space?
    I don't think it has to. I've worked out some ideas to tuck away concealed duct mini splits that feed 2-3 rooms.

    -You can't really calculate that improving air sealing by 30% will mean your room will "hold heat" 30% longer before the thermostat clicks on. A 30% reduction in air leakage does not directly translate to a 30% reduction in BTU loss.
    I know that was spurious math on my part - my intent was some sort of framing of leakage reduction to comfort levels. Probably just utter BS on my part.

  5. Paul Wiedefeld | | #5

    “PSE&G is $0.12/kwh and $0.32/therm” could you post a bill? Often they have so many charges it’s hard to sort out the true prices. $.32/therm is cheaper than the wholesale price

  6. nateflanigan | | #6

    The only bill I have is for two weeks in October and has no gas usage. Here's where I got that number - https://nj.pseg.com/aboutpseg/regulatorypage/pricetocompare
    I was looking at the first column but admittedly I have no idea what the acronyms at the top of the columns represent.

    1. Paul Wiedefeld | | #7

      Gotcha - that's the commodity price for comparison. Looks like total is around $.8/therm. The pipes are often the most expensive part. Doesn't really change much: if you want AC, you may as well pay slightly more for a heat pump, even if you don't use it 100% of the winter.

  7. Expert Member
    Akos | | #8

    I would do whatever air sealing is easy to do now and run through a winter with the house as is. It won't take much effort or cost to half that leakage rate which probably accounts for about half of your heat loss.

    When possible, dense packing the balloon framed structure goes a long way in sealing up the structure and improving comfort. This can be risky for some cladding, window and WRB details so make sure to check first.

    Once you have a baseline of energy use, you can do a much better job of sizing equipment and figuring which path to go forward with.

    Even as is, you can definitely make an old leaky structure comfortable with a good ducted mini split install. The key is to have a properly sized modulating unit, size is important as an oversized unit won't modulate and will always cycling at the low end of its output. The system also needs to have good vent placement, generally means vents along the outside walls and under windows.

    P.S. A 5 ton system already sounds oversized even with the house as is.

  8. nateflanigan | | #9

    - if you want AC, you may as well pay slightly more for a heat pump, even if you don't use it 100% of the winter.
    That has been my thinking, and that I might as well pay for a hyper-heat compressor. At least looking at equipment prices online it's only about an $800 up charge.

    Akos -
    I'm pretty paranoid about oversizing. I've had a handful of HVAC contractors come out for estimates, 2 out of have gotten back to me with proposals, the others seem to have moved on. I'm curious to see what this 5th company comes back with.

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