Hydronic radiant in floor heat temps?

[donl1150]

I live in northern Minnesota in a 3,000 SF timber frame one story rambler style home. I have 2 pump & dump FHP geo units - one 6 ton & one 7 ton. There is a 4 foot concrete floor crawl space for all utilities. The flooring in the house is a combination wood, carpeting and ceramic tile.

I am wondering what the normal temperature range should be for the water entering the loops in the floor. I know the answer is “It depends……”, but am curious as to what might be typical for such a system in my area. I think the water source incoming temp is about 49 F.

 

[Dana]

There is a huge range of temperatures that might apply. Of course "...It depends...", on the type of radiant system, the whole house heat load, the amount of active floor area, etc. It could be anything from 80F to 180F.

That said, 13 tons of heat pump seems like a lot for a 3000' house that has glass in the windows, insulation in the walls...

 

[Sponsor]

 

[donl1150]

My mistake…..the units are 4 ton and 3 ton.

As I thought, “it depends”. I just measured the water temps at the 4 ton unit while in operation. The water supply temp IN from the well is 45.5 F and water supply OUT to the dump zone is 36.8 F. The hot water TO the storage tank is 113.5 F and the return FROM the tank is 105.0 F.

I am wondering if I increase the temp to the storage tank - and that then increases the temp in the distribution to the floor - will I run into a problem with the temp of the DUMP line getting too close to freezing thus creating a new problem?

 

[Dana]

To assess the near-freezing issue would require running some of the design math. It doesn't hurt to bump it up another 5F at the buffer tank as long as you're taking notes and measuring how much that affects temperatures on the outflow.

Even 7 tons of heat pump seems like a lot for a 3000' house, but at least it's not CRAZY oversized the way 13 tons would be.

 

[donl1150]

All of this has now gotten me to thinking,…… is there a web site out there somewhere that will run an analysis that would determine which heating system would be more economical to run. My geothermal heat pumps - OR - the backup up propane fired forced air furnace.

Obviously, I would need to know the efficiencies of the units, the $/unit cost of both electricity (to run the heat pumps) & the $/unit cost of propane for the forced air system.

Does anyone know if there is such a site out there somewhere?

 

[Dana]

Unless the designer of the GSHP system was COMPLETELY inept, or unless you have a wet-gas well and processing plant on your property supplying the propane, the geo is going to come in the cheapest.

A really great GSHP designer can hit seasonal average COP efficiencies in the 4.5-ish range (all-in, pumping & air handler power included) without breaking a sweat. The more typical is 3.5, usually due to less than optimal pump & air handler selection. Real amateurs often install systems that come under 3, but lets assume it's more like 3.5. The BTU content of a kwh of electricity is 3412 BTU, so at a seasonal averaged COP of 3.5 each kwh is delivering 3412 x 3.5= ~12,000 BTU. Normalizing to kwh/MMBTU (million BTU) it takes 1M/12,000 = 83 kwh/MMBTU. The average residential retail price in MN is about 15 cents, so call that 83 x $0.15= $12.45/MMBTU.

What are your actual electricity rate, all-in delivered cost?

Propane has 91,600 BTU/gallon. Burned at 95% efficiency in a condensing propane furnace it delivers 0.95 x 91,600= ~87,000 BTU/gallon. Normalizing to MMBTU that's 1M/87,000= 11.5 gallons/MMBTU. Even at a buck a gallon that's going to be $11.50/MMBTU, and that's not counting the electricity use. This week's state average propane price is running $1.60-ish, so make that 11.5 x $1.60= $18.40/MMBTU.

What is your propane cost per gallon?

 

[donl1150]

What are your actual electricity rate, all-in delivered cost?

My GSHP is on a dual rule rate. My 12 month useage/$ for the period 9/1/17 to 8/31/18 was 17,235 Kwh / $1,166 for .068/Kwh. This is the amount to run the heat pumps, the floor distribution pumps & domestic hot woter. It also includes A/C in the summer months. My general useage bill was in addition to that. That would include a small amount of electric to run the well.

What is your propane cost per gallon?

Today’s rate for propane is $2.06.

 

[Dana]

OK, so...

83kwh/MMBTU x $0.068/kwh= $5.64/MMBTU

11.5 gallons x $2.06/gallon= $23.69/MMBTU

The condensing is roughly 4x as expensive as the GSHP. Even if the geo design was a total hack $2 propane in a condensing furnace would be more than twice as expensive to run.

At a COP of 1 (=baseboard resistance heaters) $2 propane in a condensing furnace is still more expensive than 7 cent electricity.

 

[donl1150]

Thanks for your help Dana! This confirms my thinking that I had a pretty darn efficient system. It's only on the coldest days that the propane forced air system kicks in to help out.

Back to my original question, I did talk to the installer (I am not the original owner of the house) and he told me while it is ok to try to increase the temp of the water going to the floor, the caveat is to not exceed 120 F at the discharge point of the heat pump. I could run into high limit issues and kick the unit off line. I plan to 'baby sit' the unit and increase the temp 1 degree at a time for a few days and get the temp up from the current 113.5 F to around 118 F or so.

 

[Dana]

Are there resistance auxiliary heat strips installed in this system (a common feature on heat pump systems)? If yes with 7 cent electricity and $2 propane it's cheaper/better to use some heat strip for the extra capacity rather than the propane burner. That's even more so if the propane furnace is a non-condensing 80-85% efficiency type. Even when the strips engage the heat pump is still carrying the majority of the load.

 

[donl1150]

No resistance strips in the plenum. Strictly propane fired. There is, of course, a distribution line from the heat pumps to the plenum when in the A/C mode.