We are look at putting a complete new HVAC system in a 100+ year old, 1900 sqft, 2-story farm house. Currently there is no AC. Heating is by 30 year old propane furnace to the 1st floor only. Heating for the 2nd story is by a couple of vents in the floor to let the warm air rise from the 1st floor. Needless to say, this doesn’t work very well and the $3.50/gal propane as very expensive. Our plan is to use a heat pump, either AS or GS with electric supplement.<!—-> <!—-><!—->
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According to HVAC-Calc, the heating and cooling requirements are 50kBtu/hr max heating load and 23kBtu/hr max cooling load. If I size for max efficient cooling, I would get a 2 ton unit, but that would mean I would use the electric supplemental often and diminish the efficiency of the heating system. If I size for heating, not only do I loose the efficiency when cooling, but I understand oversize AC doesn’t work very well. How do I balance those two? How much can the AC be oversized before it does a poor job of removing humidity?
Paul <!—-><!—->
Replies
Look at a two stage sysytem.
Two compressors, on small, one larger.
They will stage on as needed to meet the load demands for either heating or cooling.
Talk to your HVAC contractor. A two stage system is more expensive, but is less costly to operate.
In Ohio you are going to use a heat pump? Max heating with a HP is 32 degrees without using the heat strip. I think that you should talk with your heating contractor and see if they feel that you can be comfortable in a 100 year old farm house using a heat pump. Did a you in Morgantown a few years back and when we had the A/C put in they also put in a forced air propane unit.
For Ohio, he'd want a Cold Climate Heat Pump design. Hollowell has one. They've been run at -30 F, although down there the COP isn't great. They've reportedly been used in Antarctica. It's all in what it's designed for. In years past, air source heat pumps were used only in mild climates, where outside temps didn't get down to the 30's much and electric backup strips were a satisfactory compromise.But, a 100-year old house? What's been done to it's outer envelope recently. Those old structures would leak like a sieve in cold windy weather. One would first think that the money would be better spent in improving the shell to reduce the heat load instead of looking for a cheap source of heat to leak away to the outside. On the other hand, if the design heat load is indeed only 50K BTU/hr or so, maybe the shell has been spruced up already. The OP needs to tell us more. Did the load calc consider how badly it leaks in cold windy weather?I also have to question the assertion that two-stage heat pump designs are flaky. Where'd that come from, some combustion-based heating system vendor?
also have to question the assertion that two-stage heat pump designs are flaky. Where'd that come from, some combustion-based heating system vendor?
Thank you Dick !
Multi-stage commercial systems have been around for years. Same manufactures as the residential market, so applying existing technology from one area to the other was a no brainer for them. In fact it let them slide for a few extra years with the COP and SEER numbers they were suppose to achieve. Now, they are getting better.
"For Ohio, he'd want a Cold Climate Heat Pump design. Hollowell has one." Unfortunately, the closest Hallowll dealer is over 100mi away.
"But, a 100-year old house? What's been done to it's outer envelope recently. Those old structures would leak like a sieve in cold windy weather. One would first think that the money would be better spent in improving the shell to reduce the heat load instead of looking for a cheap source of heat to leak away to the outside. On the other hand, if the design heat load is indeed only 50K BTU/hr or so, maybe the shell has been spruced up already. The OP needs to tell us more. Did the load calc consider how badly it leaks in cold windy weather?"
At some point vinyl siding :( has been added with 1/2" sheathing underneath the appears to be compressed fiberglass impregnated w/asphalt. I think it's only about R-1.5, but it should help quite a bit with the infiltration. All the windows have been replaced with double pane. I posted the results of the blower door test in my previous message and out experiance this winter would lead us to belive it's pretty accurate. Not much in the way of cold drafts. The load calcs were done considering the additional sealing that will be done around the rim joist and from the attic.
Too bad the nearest Hallowell dealer is too far away to suit you. Hopefully you can find someone close enough who has a good HP with a good rep and which will work with air temp down to low single numbers without needing electric heat. If it needs the backup electric when it gets below freezing, when you need the most heat load, I suspect you'll be disappointed.Are you in a rural area, with a drilled will for your water? Ground source heat pumping has a generally higher COP for cold climates, due to the relatively constant water temperature, likely in the upper 40s to low 50s in Ohio. For standing column well setup, you need on the order of 80-100 feet of water column per ton of load. For you, probably a 5-ton unit would be what you'd need, so that would mean 400-500 feet of water. If your well isn't that deep, then you'd have drilling costs for a deeper well or parallel wells, and that runs up the installation cost. I suspect the cost-effectiveness for GSHP is best for a very well insulated house, with low heat demand, in a rural area where the well is needed anyway.COP for that would be 4.0 or better, all the time, vs. maybe 2.5 or so and declining with outside temperature for air-source heat pump. But I imagine you've already explored all of the technical stuff on your options.
"In Ohio you are going to use a heat pump?"
Heat pumps are pretty common in the area. We've gravitated to the heat pump because my other choices are propane and oil. Propane is over $3.50/gal and from what I understand oil isn't any better. It will have cost over $2000 to heat this winter. I don't have a crystal ball, but I don't see that getting any better in the future. Electricity is reasonable around here and hope (knock on wood) it stays that way. We have coal powered plants and coal is still quite plentiful. Also, if in the future solar becomes cost effective it can be applied to our heating and cooling along with our other electricity usage.
Paul
I did a comparison between propane and electric:
1gal propane = 91,500BTU
(Assuming 100% efficiency)
91,500BTU = 26.8kWh
At current electric price of $.10/kWh, propane would have to be <$2.68/gal to be more cost effective. Even less when you factor in efficiency. We would have been better off this winter turning off the propane and plugging in electric space heaters!
Paul
black46
I realize this seems a little backwards but consider going with the proper sized heating unit and accepting a too small A/C unit supplimented by a couple of small window A/C units in the less frequently used rooms.
The reason is much of the time you won't need the full use of A/C and those really hot days where you need added capacicty the small window A/C units are cheap and probably as efficent or more efficent than that large unit you are installing. My whole new house (5500 sq.ft. is cooled with 5 small window units installed thru the walls (so I don't lose the view) Only rarely have all 5 units been on at one time..
It is surprising that a 100 year old farm house is sealed and insulated that well. Will it be possible to keep the air handler and duct work inside the conditioned space? Have you considered having a blower door test done? You need to look at both factors before deciding on the size of the unit.
HVAC-Calc can tell you how many BTU's you need of heating and cooling, but it doesn't tell you how big the system needs to be to actually deliver that many BTU's. A 2 ton heat pump will deliver 2 ton of cooling at laboratory conditions, but not in the real world. You would probably need at least 2-1/2 ton of cooling to deliver your 23kBtu/h.
Another possibility is to go with a ductless system. Sanyo, Fujitsu, and others make some very nice units that deliver good efficiency without requiring major construction issues to install the duct work. With multiple indoor units running off one variable speed outdoor unit, it makes for a very comfortable system. These systems are not cheap, but if you factor in the cost of adding the duct work, they could end up being more cost effective.
the two compressor systems are complicated and have not been well accepted by customers.
"It is surprising that a 100 year old farm house is sealed and insulated that well. Will it be possible to keep the air handler and duct work inside the conditioned space? Have you considered having a blower door test done?"
We did have an energy audit w/blower door test done. We were pleasantly surprised with the results. The infiltrations results were .6ACH summer and .8ACH, which is slightly below average. We should be able to improve that a bit fairly easily. The attic has 8" of blown in insulation. We will be upgrading that as well, which may give us the opportunity to make the attic conditioned space for an air handler and duct work.
Paul
(1) You better pay attention to the sensible and latent loads for your air conditioner, and those same specs for the unit.
(2) The only way to address the need for more heat than cooling in a heat pump is with a multi-stage unit. Again, pay attention to the specifications. The Hallowell unit is the only air source cold climate heat pump on the market at this time. It has 3 stages, and it would work fine in cooling. All of the specifications are on their website.
As you've discovered, heat pumps are not the 'cat's meow' that some claim them to be.
It looks like you're going to want some supplemental equipment. For example, means to control the humidity. Use of separate heating / cooling equipment for different parts of the house also often makes sense. What doesn't often make sense is expecting one machine, on thermostat to do it all.
As a general approach, I'd go for the small unit, use a gas supplemental heater, and look for ways to reduce that heating load. besides the obvious - sealing and insulation - improving/ controlling air circulation in the home can make a huge difference.