Calculating when it makes sense to use heat pump and when to use steam boiler

agurkas

I just got my 1st gas bill for 1 month and it is $550. Bit shocked. Granted, this is my 1st winter in this house. I may have 3K sq ft Cape, I only heated like 2/3 of it, since some room I just keep cold.

I have brand new Mitsubishi Mr. Slim heat pumps (2X 6K heads and 2X 9K). COP on the system is 2.6, so that would be what like 260% efficiency. I am yet to use them, because I was told in these forums that it is cheaper to run gas boiler.

Boiler is converted Weil McClain WGO4. Which I think is 75% efficiency (I think). Feels bit optimistic number for something that is 10yrs old, was built for oil, and now has Carlin gun.

But now that therm runs $1.40 for gas and electric rate is 19 center per everything included, I wonder is it really cheaper to run gas heat at least in above freezing temps. I am struggling to see how running a boiler from 140F start (indirect water heater is attached to it, hence semi cold start) for 30-45 minutes to get 3-4 degree bump in 3 bedrooms vs running heat pumps is more economical.

Jamie Hall

Check the manuals for those heat pumps. That will give you -- or at least should give you -- an indication of how their performance is affected by air temperature. There will be a minimum temperature at which their performance drops off significantly; they may have electric resistance back up heat -- which you don't want to use.

agurkas

There is no backup resistance heat on those units (at least I don't see anything in the specs). Condenser is Mitsu MXZ-2B20NA and head units are MSZ-GE06NA and MSZ-GE09NA.

They are rated into -7F territory, though at 17F is where it looks it drops.

Still scratching my head how running boiler for all that time is cheaper than running heatpump.

SWEI

There should be tables in the technical manual showing BTU output, COP and kW consumption for various operating conditions. SWT matters just as much as OAT does.

agurkas

SWEI,
Not even sure how to extract what you brought up from graph below. That is what I found in service manual.

agurkas

COP at 47F is 3.77. So would that make it past break even?
Still getting confused in that alphabet soup.

I am also thinking about when you don't have to fire up the boiler, I am not dealing with heat loss of 200 feet worth of 1.5" to 2" mains (even if insulated with 1.5" insulation)

Spence

There are two balance points with which to work regarding hybrid systems; thermal and economic. The TBP tells us at which ODT the heat pump output exactly matches the heat loss of your house. At any condition below this point, the HP needs supplemental heat from the boiler.

The EBP tells us when the cost of using a fuel is not fiscally responsible. It would make sense to determine the TBP from your load calculation, find out the HSPF of the HP, and have a combustion analysis performed on the boiler. This way you're not guessing; you have real numbers with which to work.

Mark N

How many total therms were used? What was your heating usage and non heating usage? I think I remember you also have a Hot air furnace to heat part of the house. How much gas did that use. When it is above 40 it might be less expensive to heat with the split minis. My neighbor had an addition done a few years back that is heated with split minis. I've never asked what his electric bill is like when its cold.

Zman

Have you used this one? www.eia.gov/tools/faqs/heatcalc.xls
I is a quick way to do the comparisons.
In your case it is surely cheaper to run the heat pump on the warmer days. The question is, at what temp do you change over?
If you don't mind the extra effort, you could log both meters daily and start experimenting. You will need the actual heating degrees (diff between indoor and outdoor temp) as well in order to adjust the data.
Carl

agurkas

Zman,
Yes, I used it. Formulas in there assume heatpump is primary with resistance backup. Not the case for me and actually makes huge difference in calculations.

agurkas

Mark N,

I am in Boston and it was from 12/10 to 1/15. There were some cold days, but still...

I am only heating 2nd floor with steam or heat pumps. 1st floor, where the steam heat is, I have turned off and those rooms usually sit at about 50F.

My walls on the 1st floor are empty (just discovered it couple weeks ago). 2nd floor I got recently attic air sealed and blown in. Also half of the all west walls are knee walls, so those got densepacked. Short north and long (60') east wall are empty.

Heatloss my HVAC guy calculated is no longer valid, since we thought there was at least minimal insulation in those walls.

What makes calculation harder is that I have 900sq ft (20ft vaulted ceilings) I am heating with 97% efficiency condensing forced air.

All I have for reference is that previous owner spent $6K in oil in 2013-2014 heating season. But she had original windows from 30s, very poorly insulated attic, and house was really drafty.

agurkas

I am losing a lot of heat through those darn walls...

agurkas

I am going to seal up the crawlspace (need to do it anyway for radon) and make it unvented. Probably went through two dozen tubes of caulk already. Chasing leaks everywhere. Sealed up one of the two fireplaces.

Mark N

Don't you have Nest or Ecobee thermostats? Don't they track run time? How many hours did your boiler and furnace run for the billing cycles?

agurkas

I do have those. Nest keeps only 10 days, so that is worthless.
Ecobee I can pull data out, but it isn't really where my concern is.
I know I have insulation problem that I can't fix this winter. Contractors I like (who did my attic) are booked 3 months solid. I have vented the heck out of my mains and insulated all of them too. Modified pressuretrol too to not go above 1.25PSI (which removed gurgling sound from one of the mains).

So only two areas I have I can improve and probably see something beyond marginal - running heat pumps only in bedrooms we use during the night when weather is above 40 and going nuts with Great Stuff and caulk guns.

Mark N

Insulation will always help. But don't you think it would be nice to determine how much gas each appliance is using, also need to separate out your non heating gas usage. From 12/12/14 to 1/15/15, I used 209 therms of gas, of which 178 was for heating. My house is half the size of yours. Those 209 therms cost $126.42.

agurkas

Mark N,
We use gas only for heating (indirect water heater is attached to steam boiler). We aren't using gas for cooking, since we have really tough venting code.
I probably try to measure how much boiler is using and furnace separately.
I know last night in minisplits cost me $2.25 and gas furnace and water heating cost me $12. Did simple took picture of meters before I went to sleep and then again today in the morning.

Mark N

Your furnace and boiler used 8.5 therms overnight? How cold was it? I doubt there was much demand for hot water overnight.

agurkas

Going to try to do the same tonight. Not trusting my gas meter reading skills yet.

bio_guy

A few points to remember and some factoids:

Respondents have done a good job calculating the cost of a BTU from your gas-fired source vs. HSHP. The complicating factor is the fact that the MSHP cost is a moving target that varies with outside temp, but I think the answer is clear, gas.

MSHPs do a good job of heating in cold weather, higher than the AHRI ratings because the AHRI system requires the compressors to be run at constant speed. (That is probably the 3.77 @47 F.) Real output is better than that. The FE (Hyperheat, h2i) series is better than the OP’s, but they are still very good. Keep in mind that the overall lifetime operating costs are probably lower for a boiler/hydronic system than for a heat pump based on equipment costs so even if energy costs are even, you’d be better off heating with the boiler.

Some MSHPs have resistance heat. I understand that this was vetoed in the USA due to plastic housings, but I have not verified that. Given that is true, foreign markets may have them and a few may have slipped into the USA.

Some useful links/references:

Long-Term Monitoring of
Mini-Split Ductless Heat
Pumps in the Northeast
Building America Report - 1407
December 2014
Kohta Ueno and Honorata Loomis
[Note that I think that their statement that running mini-splits flat-out is over generalized. My examination of the cited paper (below) indicates that this is true for Fujitzu but not so much for the Mitsubishi.]

Laboratory Test Report for Fujitsu 12RLS and Mitsubishi FE12NA Mini-Split Heat Pumps
Jon Winkler
National Renewable Energy Laboratory

http://usa.mylinkdrive.com/uploads/documents/4319/document/22_2013_P-and_M-Series_Engineering_Application_Notes.pdf

agurkas

At this point probably I should borrow CT current logger and just overlay that data for couple days. So many darn variables.

unclejohn

I would start at shutting down the heat pumps at 35*f Then next month try 30*f. Trouble is you have no way of setting up the system do make a automatic change over.

Spence

An isolation relay makes an easy automatic changover.

Rich_49

WWSD @ 47*F . Done . Boiler comes on at temps below 47* , heat pump won't run because it won't see a need . Simple .

unclejohn

Its four heat pumps as I read. Not saying it can't be done but it's not easily done. At 47 the heat pumps are kicking butt.

Rich_49

Any COP less than 4 cannot be considered kicking butt .

bio_guy

Rich said:

Any COP less than 4 cannot be considered kicking butt .

That would depend on what the cost of the alternative heat source is, and the temperature differential it is working against.

agurkas

Rich,
I think you are thinking ground source not air source heat pumps. According to most of the stuff I found, HE ASHP get to seasonal average of 2.8. 4.0 is achievable in mild weather.