BTU kW Sizing - I am pretty sure this is a stupid question but you decide!

Prevch

I currently heat my Northern Wisconsin home with the EB-MX-20 by Electro Industries. It is a 20kW 68K BTU electric boiler that provides hot water to radiant loops throughout my home. I am looking to replace the system as my main heat source with an air to water heat pump, but I am uncertain on the size of the system that I should pursue.

As it stands, my current electric boiler has no problem at all heating my home. Even on the coldest days, it keeps up with demand just fine. The boiler has 8 total temperature presets ranging from 90 degrees up to 180. The majority of the time, I have it set to 126 which is preset 4. though in the depths of winter, I do sometimes push it up to the next setting which is 138 preset 5. In other words, I am using roughly 75% of its total output power.

My question is this, given that I am only using approximately 75% of the potential output power of the electric boiler even on the coldest days, do you think I could safely pursue a heat pump that is 15kW? I still plan on keeping the electric boiler for auxillary heat in the event that the heat pump can't keep up, but I don't know if I am thinking about this all wrong. I hate to buy too large of a unit and waste electricity, though being undersized and incapable of keeping the house warm would be worse.

If you have any guidance or resources that you think would be helpful, please let me know. Thanks!

Jamie Hall

There are two different aspects to this. The first in response to the specific question is that heat pumps are usually (at least in the US) rated in BTUh power, not in KW. Thus you want to compare your boiler power in BTUh to the rated output of the heat pump, also in BTUh. Since you know that your current boiler is more than adequate to meet the house demand, you certainly don't need to go larger — and might or might not be able to go to a smaller unit.

However.

Unlike a boiler, the potential power output of a heat pump falls off at low temperatures, so you also need to do you power comparison using the heat pump output at the lowest temperature you want the unit to provide your heat. My recollection of northern Wisconsin is that it can get a bit chilly, so this will be an important factor. The temperatures at which various heat pumps start losing power, and how much they lose, varies a lot with different models and different makers.

mattmia2

Note that you are just changing the temp setpoint of the boiler. It could be using the full input of the boiler to make the water that temp. You don't know without measuring the power it is consuming over time.

Hot_water_fan

use this: https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

is this floor heating? If so, those temps are extremely high. Why are you operating it like that?

Caleffi’s Idronics journal on air to water heat pumps is a wonderful resource. You have a pretty easy swap here (assuming radiant floors), so that’s exciting!

Kaos

See above link on how to get actual load. Convert you monthly kWh usage to Therms by multiplying by 0.034121. Electric boilers are 100% efficient. Fuel use based heat load calculations are pretty accurate assuming you have reasonable window sizes.

Boiler output depends on building load. Increasing the temperature of the boiler won't effect it much, if you crank up the target temperature the thermostat will cycle the zone more to match the two.

I'm guessing you are somewhere in zone 6 or zone 7. There even cold climate air to water units will struggle unless you can get the emitter temperatures low. You'll have to either oversize to cover cold snaps, spec ones with built in resistance coil or keep the existing boiler as a 2nd stage in case the air to water unit can't keep up. Most have controls for enabling an external boiler.

Make sure to keep the air to water unit well off the ground and away from walkways. By the time February is done, I have a quite a nice skating rink around the unit from defrost.

Prevch

https://forum.heatinghelp.com/discussion/comment/1857448#Comment_1857448

Thank you for that input, that is exactly why I asked! I really appreciate your insights.

Prevch

@mattmia2 I actually recently purchased a home power monitoring system for individual circuits so I can do that. Thank you!

Prevch

@Hot_water_fan Thank you for that resource! Yes, it is in floor heat. The reason that I am using those temps is because that is what it takes to heat my house. The return temp is about 10-20 degrees less than the numbers I gave. My floor system is unconventional in a variety of ways. The builders of the house did not properly install the radiant floor heat in the basement. I later retrofitted the rest of the house with in floor heat in a somewhat unconventional manner, so it is kind of a hodge podge system. That and my house has super tall ceilings and gobs of windows. It looks nice, but is costly to heat, hence the desire for a heat pump system.

In any event, I am excited to hear that it is a fairly easy swap. A part of me is looking forward to the project. The other part is just wondering how far overbudget I will go lol.

Thanks again

Prevch

@Kaos Very good point on keeping it away from walkways. I am glad you said that because I would have made a mistake.

Yes I plan on keeping the boiler as a backup heating unit. It can get crazy cold here, but under normal circumstances, I am hopeful that it will help to reduce cost.

mattmia2

If the system is undersized or the loss is something that no radiant system can cover with the available floor area, you might want to add some panel radiators or something to supplement it.

hot_rod

How warm does you floor surfaces get with that high supply temperature.

Most A2WHP have back up electric resistance elements, should the HP not maintain temperature.

Do you have enough power in the panel to run a HP and back up boiler?

Do you have AC or a ducted system also?

Prevch

@mattmia2 We have quite a bit of floor area, so should be good, but certainly add if necessary.

Prevch

@hot_rod I actually am not sure how warm the floor itself gets. I would have to measure to be sure. Certainly not hot, but comfortably warm.

I have 200 amp service, so I should have enough. I do have a ducted system as well. I generally only use it for AC, but it does have a heat strip in it in the event that I want to use it for heat.

Hot_water_fan

@Prevch If you're using 140F water for a floor, you're extremely short on heated floor area! You'd typically expect around 100F or less. You gain efficiency with a heat pump as the supply temp decreases, so you can use the higher temps, just only use them as needed (ie use an outdoor reset curve or similar).

The next question would be related to the ductwork. As an air to water heat pump can heat and cool, you can use the ductwork and an hydro air handler to cool. Or use an air to air heat pump to cool and heat.

Prevch

@Hot_water_fan Well in the basement, that is likely true. The original home builders did not put enough hose in the concrete and we also believe that it is not at the correct depth nor is it properly insulated. For the rest of the house, we are essentially using all of the floor area. What prompts the increase in temps is that the basement can't keep up. The rest of the house could work on a lower temp.

We prefer the comfort that the in floor heat provides, so I likely would not do an air to air heat pump, but I could see a scenario where we eventually used the A2W heat pump to run the AC

Hot_water_fan

Gotcha. In that case, run the basement on the higher temp but make sure the rest of the house uses lower temps!

Prevch

@Hot_water_fan I think the only way to do that would be a mixing valve correct?

Hot_water_fan

https://www.caleffi.com/sites/default/files/media/external-file/Idronics_7_NA_Mixing%20in%20hydronic%20systems.pdf a few ways to do it. The goal is to use the lowest temp as much as possible and only increase as needed instead of producing high temp water then mixing down

Kaos

With air to water you never want to be mixing water down, it is a big hit in COP. You want your emitters to run at the temperature the AWHP puts out. Baseboards are not the best supplement as they don't put at all that much heat per foot at the low SWT. If you need more heat, the best option is a low temperature panel rads.

As for cooling, the idea of using the air to water unit sounds good until you dig into the details. The bits you need are expensive and EVERYTHING not properly insulated will sweat. Since there is an extra conversion, efficiency isn't all that great. About the only good thing about is you can micro zone. I would stick to regular air to air units for cooling.

hot_rod

If you have staple up or suspended tube for the upper floor you may indeed need that higher SWT. Or carpeted floors drive up the SWT requirement.

A room by room load calc would tell. But you already know what it takes.

Although floor heat and forced air behave much differently, A large hydronic coil in the air handler could allow you some heating supplement at low SWT, 120 ish. And some cooling.

Is peak efficiency the main driver in your decisions?