Can I run a spacepak heat pump system concurrently with my two-pipe steam boiler?

Jamiedle

We moved into a 95-year-old house with a two-pipe steam system last March. Our gas bills so far this winter have been very high, so my question is whether we could run our new heat pump at the same time to save on gas costs.

Some background: The boiler was "tuned up" by a reputable contractor (Supreme Heating and Cooling in Detroit) in the fall, including replacing end-of-line vents. The system works fairly well, although we have a couple of radiators that don't heat properly and some water hammer. We've been quoted something like $850 per radiator to replace valves and traps, and we plan to do that one by one eventually, but don't want to spend all of that money right now.

We also replaced the thermostat for the boiler with a simple one that has a boiler setting on it, which I believe helps the boiler to cycle less often.

The house had no air conditioning, so we had a SpacePak system installed in the late spring last year, which includes a Bosch heat pump. We did use the heat pump a few times in the fall before we got the boiler going to take the chill off at night.

My question is whether we could try to reduce our gas costs by running the heat pump at the same time as the boiler. We understand that turning the thermostat down intermittently (for example, at night or if going away for the weekend) is not a good idea, and I'm not sure how the two systems would interact with each other. The thermostat for the boiler is in the living room, on the main floor, and one of the spacepak vents is almost directly above it. The thermostat for the heat pump is in the upstairs hallway. One note here: that thermostat is directly under the return vent which meant that when we used it for heating in the fall, it registered the cold air flowing into the return and never really cycled down.

I have a feeling that this is just a bad idea in general, but on the chance that it's not, I'm asking the experts here. Thank you!

ethicalpaul

We understand that turning the thermostat down intermittently (for example, at night or if going away for the weekend) is not a good idea

It's really not a bad idea as long as you are aware of the possible issues. The boiler really doesn't care.

The way I would do it maybe is have a thermostat that can cut over from one heating system to the other one depending on the outdoor temperature.

This is what I have set up at my mother's house where she has a heat pump and an oil furnace. I have hers set to go to oil below 30F.

The pros will weigh in with their opinions.

Jamie Hall

Not a bad idea at all. Quite a number of folks who have both boilers and heat pumps do this. Usually it is a matter of running the heat pump when it is a little warmer out — depending on the heat pump — and it is running as efficiently as it can, and reserving the boiler for colder days. Other folks run the boiler to maintain a base temperature in the building, and use the heat pump to warm specific spaces or take the edge off. Lots of strategies which work.

Whether they save any money or not — that get's complicated.

Now in your situation, though, you ay have some difficulties because of where thermostats are located — they may not really reflect what is happening, or what you want to happen. You might want to consider relocating one or both of them.

The other aspect of this is what the thermostats are set to. I personally would suggest setting the boiler thermostat at some moderate to low temperature, and letting the boiler handle the base heating load — no setbacks, just holding some temperature. Then use the heat pump thermostat to bring desired spaces up to some higher temperature when and as needed. Forced air — which is basically what a heat pump provides — doesn't mind being ramped up and down as much.

hot_rod

Some of the heat pumps are dual fuel, mine kicks on the gas fired furnace at around 50°

The forced air heat pump heat will not be near as comfortable, as steam radiators, but possibly less $$ to operate.

The home dictates the heat needed, so looking at the two fuel costs could give you an idea of the least expensive option.

Plug in your fuel costs here. If it is an older steam boiler efficiency could be below 80%

The heat pump efficiency is based on outdoor temperature, the manual may indicate COP at different outdoor temperatures, plug that number in tot he HP column.

https://coalpail.com/fuel-comparison-calculator-home-heating

DCContrarian

Before doing anything you should look at fuel costs. A heat pump is not intrinsically cheaper to operate, in much of the country it's actually more expensive. You don't say what part of the country you're in, but electricity costs vary enormously around the country — like a by a factor of ten or more.

DCContrarian

@Jamie Hall : "I personally would suggest setting the boiler thermostat at some moderate to low temperature, and letting the boiler handle the base heating load — no setbacks, just holding some temperature. Then use the heat pump thermostat to bring desired spaces up to some higher temperature when and as needed. Forced air — which is basically what a heat pump provides — doesn't mind being ramped up and down as much."

The only thing to look out for is whether the heat pump has supplemental resistance heat. If it does, it will typically kick in if the room temperature is more than a couple degrees below the thermostat setpoint. The supplemental heat is going to be much less efficient, like three times as costly to run.

hot_rod

Do you have the high velocity system from SpacePak? You can probably find efficiency data at their website.

As outdoor temper drops efficiency drops and so does capacity. But it still could be more efficient than an old steam boiler. Especially with unknown efficiency. I doubt the boiler gets over 84% ever? It could be in the 70% range also! You could nail down the SpacePak info fairly accurately, but not the boiler.

So the operating cost comparison is not so cut and dry.

jumper

»and letting the boiler handle the base heating load«

Ideally one would downsize boiler and install orifices on radiator inlets? In many locations design temperature days are small fraction of heating season.

hot_rod

Typically a dual fuel system has a controller that watches outdoor temperature. So if you used the HP until it falls to say 45° outdoor, then the boiler takes over.

When the temperature outdoor rises to say 50° the steam would drop off and the HP takes over. The switch over temperature could be determined from the HP data when the COP drops too low, switch to the boiler.

I know SpacePak has s SSIC control that could do that, or the HP you have may have it built in already.

Past fuel oil bills could help you determine the operating cost comparison.

While electricity may be $$ in your area, it is probably consistent, whereas fuel oil can vary, even month to month. In 2022 fuel oil went over $6.00 per gallon to some customers, what are you paying today?

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1882270#Comment_1882270

True on the design temperature days being rare… but… that's exactly when you are going to be depending on the boiller and the radiation to be working at its true full capacity.

May I make an analogy? Back in the days when I trailed horses all over the east, my tow truck had the biggest V8 that Chevy made (my son-in-law's truck is now the same idea). 90% or the time I could have gotten away with a smaller engine (5.7 vs 7.4). That remanding 10% of the time? Have you ever played tag with an underpowered 18 wheeler on some hilly interstate? Trust me — the guy or gal driving the big rig isn't any happier than you are.

pecmsg

I run my heat pump to 35* outside

Lower then that the hot water takes over.

ethicalpaul

True on the design temperature days being rare… but… that's 

exactly 

when you are going to be depending on the boiller and the radiation to be working at its true full capacity.

I still have yet to see a heating system anywhere that couldn't keep up with the outdoor temp, no matter how cold it got. It seems that there is so much extra slop in the sizing calculations.

and if it can't keep up during the very coldest nights, that's what sweaters and blankets are for. What's it going to do, let the temp drop a few degrees? Both an "underpowered" truck and an "undersized" heating system will still get you there.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1882386#Comment_1882386

it was 2023 when HH had a lot of posts about heating systems not keeping up. I think 2015 and 2018 had some cold snaps also, back east

You must have been hiding under blankets 😉

From 2023 weather report:

ethicalpaul

<Shrug>

I will say I am surprised that Boston, where I keep hearing from them that they have horrible winters, are shocked by a -10 overnight temp that happened once in over 67 years. What are they even complaining about every year?

Jamie Hall

I have to confess, @ethicalpaul , that I have never quite managed to understand why one would deliberately underdesign something. Whether it is a heating system, or a bridge, or an airplane. Gross overdesign, yes, I do think that is silly (and sometimes counterproductive), but deliberate underdesign? There may be a small capital cost advantage to underdesign, but not an installation difference (unless the underdesign is large, in which case, at least as an engineer, you should lose your license) and the running costs will be very similar.

So why the hair shirt?

pecmsg

https://forum.heatinghelp.com/discussion/comment/1882391#Comment_1882391

Because its Boston?

ethicalpaul

@Jamie Hall I guess it's a question of terminology. How can something be undersized if what's considered appropriate sizing is actually oversized? I know weather has variety and people get freaked out if their system even runs at 50% duty cycle so it's just a discussion.

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1882270#Comment_1882270

You can not orifice radiator inlets on a steam boiler radiator as you can on a hot water boiler radiator. This advise is incorrect and confusing since it is true for some situations but not this situation.

Jamie Hall

I think it's not so much a terminology difference as a philosophy one. Engineers have been commented on from time to time on The Wall — usually laughed at for a lack of real world experience (with some justice). But engineers have a mental quirk. If you give them a set of design parameters, in this instance a design outdoor temperature and a minimum maintained indoor temperature, if they then design something — either through negligence or philosophy — which they know or should have known is not going to meet those design parameters, then that design is, bluntly, a failure. Simple as that. Engineers — at least good honest ones — won't accept "well it works most of the time". Either it works as intended, or it doesn't.

EdTheHeaterMan

@ethicalpaul is a homeowner that goes above and beyond when it comes to understanding how his steam boiler operates. Although his boiler is not like everyone else's boiler, there are some similarities. It is piped to a very old heating system of radiators.

As a professional that has been to over a thousand heating systems in my 45+ year of experience, I believe that Paul will yield to my expertise as his experience with other people's heating systems is somewhat limited (although I applaud his efforts to help others with his consulting side hustle) when it comes to many heating problem solutions.  Where we differ is on how to heat DHW.  His dislike of indirect water heaters and my inexperience with HPWH puts us at odds on that subject on a regular basis.  

But to the point of undersizing; there is more than one way to undersize a system.   I remember one elderly widow who wanted a price to replace her coal converted oil heater because she thought it was not big enough.  When I arrived she had two other quotes in hand to replace the boiler.  What I saw was an oversized coal boiler (not undersized at all) connected to a loop of baseboard heaters. 

The baseboard heaters were also not undersized.  There was probably too many feet of baseboard heat to the size of the home.  The loop ran the perimeter of the first floor all the way around to the boiler room where it went to the second floor and wrapped all the way around to the front of the house and back to just above the boiler room where it dropped down to connect to the return. Over 100 feet of baseboard element on one loop.  Can you see what is undersized here? 

The boiler replacement quotes were all $2,700.00 and $3,200.00 and I believe those new boilers would not have solved this customer's problem. I quoted $500.00 to add about 12 feet of 1” pipe and split the upstairs and downstairs into two separate loops with about 50 feet of baseboard on each loop.  There was a ball valve on each loop included to facilitate purging and balancing the system since there was only one thermostat for the entire home.  Undersizing pipes can be a reason your system is undesized.

Another customer with the same problem called me every time the temperature dropped below 5°.  This did not happen every year, but every couple of years since the temperature didn’t get that low every year.  Their undersizing was a result of the carpet installer putting extra padding on the wall to wall carpet all the way up to the baseboard radiators.  The radiators worked fine unless the outdoor temperature got too low.  With the restricted inlet space to allow enough air flow into the radiator element, the radiator was not giving out all the heat it was designed to produce.  I call that “Undersizing by interior decor design”  There was another floor heat system that would not heat the rooms as designed.  The homeowner did his own staple up Pex tubing to save $$$ and the contractor installed the boiler and manifolds.  Since it was oil heat, I was called to do the annual maintenance.   Every year the homeowner complained about the bedrooms being too cold.  Every year I told him to finish the basement ceiling insulation because it was much too hot to work in that basement and that would put that heat into the rooms above.   

After 5 years of not insulation, he was getting a divorce and part of the divorce settlement was to finish all the unfinished projects.  The insulation was completed and guess what… the rooms heated properly.  I call that “undersize by laziness fixed by decree”.  

But Paul is correct about boilers rarely being undersized.   By design when the load calculation is completed, there is a lot of oversizing included when a wall is 12’ 5” we often just plug in 13’ to make the math easy.  When we get to the end result and the NET rating of a boiler is 40,000 BTUh and the load calc requires 41,000 BTUh we often select the next size larger just to be safe. We just don't always pick the correct pipe size when installing it.

ethicalpaul

https://forum.heatinghelp.com/discussion/comment/1882406#Comment_1882406

Well it is 2-pipe so I think orifices or other inlet adjustments are possible.

Thanks for those nice replies above @EdTheHeaterMan and @Jamie Hall

Jamiedle

Thanks for all of these comments so far! To clarify a couple of things: Our boiler runs on natural gas. Both gas and electricity are very pricey here in our Detroit suburb.

Anyone who comes in looks at our boiler and says it's huge, even for our 2600 square foot house. (Input BTU is 280K, heating capacity 231K, if the stickers are to be believed.) The yellow sticker says it's 81.4% efficient, but it's 23 years old. We haven't had a day in Michigan so far where the steam system couldn't keep us comfortable. It's the almost $400 monthly gas bill that brought me here to ask the question!

I have no idea how we would change the controls of either system to respond to outdoor temperature, but we could probably find someone to help with that. It sounds like running the SpacePak heat pump at the same time as the boiler may or may not save money.

DCContrarian

What are your current rates for gas and electric?

jumper

What is time frame? How long will you have this house? Payback takes time.

hot_rod

ask around to neighbors with similar sized homes

The more important number is how much fuel 400 bucks buys

Most utility providers allow you to search past fuel consumption. Gas providers often raise costs and add other “fees”

You bill should break all that down

EBEBRATT-Ed

So Boston was -10 and they all complained. I am sure all the MIT professors were cold. Put a blanky on. I live in Western, MA which doesn't exist as far as Boston is concerned the western border of the state is Worcester. We design for 0 here.

I don't know what Boston's design temp is but its not -10. I think they are warmer than Springfield and I know Worcester is.

Heat loss calculation never consider your refrigerator condenser output or you range output or people or lights etc etc.

One interesting thing I just found out in the new @2026 NEC is that when doing an electrical load for anew house the first thing you do is calc the square footage.

For the past 80 years?? they used 3 va /square foot for a house. This covers all general use receptacles and lighting. They you add dryer, range, Ac or heat (whatever is larger), dishwasher, disposal, washer and kitchen receptacles etc

Because of LED lights they went down to 2 va/sq foot for the load calculation, but the service entrance is still figured at 3va/sq foot.

Sound like CYA to me.

offdutytech

@Jamiedle I'm in the Detroit area. If you like give me a shout and we can take a look at the system to see what controls would work.