Wood boiler/heat pump combo

[dregier]

Planning a new home build and I'm completely new to hydronics. I've been getting lots of great info from the posts on this site and figured it was finally time to ask a question. I'm wondering if anyone on this site has experience connecting a wood boiler and an air-to-water heat pump on the same system.

Here's the plan. Radiant in floor throughout the house, garage, and shop. Low temp design (say 110ish?). I'm planning to use a Froling FHG 40/50 with 400 gallons of storage (lots of firewood on my 48 acres). Air-to-water heat pump with electric backup for heating in the shoulder season and when I'm not around to feed the Froling. AWHP also provide summer cooling with high velocity air handler.

I can't seem to find a good way to hook the systems together. I don't want/need glycol in the froling/400G side but need it on the AWHP side so a heat exchanger will need to be on one side or the other. The AWHP needs a buffer to prevent short-cycling but 400G of buffer is more than a little too much. I thought a good solution would be to connect the AWHP with electric backup boiler through the upper 1/4 of the 400G tank so that it is only buffering with about 80 gallons of the tank. The Froling would be connected top to bottom to utilize the full volume. Would this work?

My other thought is to set up the AWHP with a small buffer tank controlled at the design temp and heated through heat exchange with the wood boiler when it's running and then the AWHP would take over when it isn't. The heating load would then come off the small buffer tank.

My goal is to keep the design and controls as simple and efficient as possible. I'm spinning my wheels and would love to hear from anyone who has had experience with this combo. Cheers.

 

[Bad LP]

Being your plan is a back up/shoulder season system why not run duct work for your AC using a gas or oil fired system?
The cost is going to be low compared to your radiant side.
Many will state use split systems but they don’t work in extreme cold. My proof is 6 miles south of me for a restaurant that froze up solid in below zero teens weather under 30 days ago.
You couldn’t pay me to use a glycol system in anything I own. I managed a single estate with 3 houses on glycol so I speak from direct experience.

 

[EbS-P]

How many square feet in house garage and shop? First thought is if you want ac you are going to need ducts. Seems simpler to have an air to air ducted heatpump separate from the boiler. Attic ducting wouldn’t bother me as much since it’s not going to be used too much in the winter. Variable speed compressor with two or three zones. Or just a couple mini split units.

Now I totally get the radiant heat but how often do you envision switching back and forth? (Do the two systems need to be integrated?) I see the heatpump running until temps drop consistently below freezing. if you want your shop or garage heated why not a mini split? Will you want humidity control in the shop?

I don’t think AWHP tech in North American is ready for prime time. Biggest reason is knowledgeable installers And technicians. I do think you would use the heatpump more in an integrated system. For instance our weather the past week has been near freezing at night up to the 50s. Fare at night or early morning and heatpump in the afternoon. But that could easily be done with separate systems too.

Based on my wood heat and heatpump combo wood Handels 75% or more of my heating. The cheapest way to install the heatpump would be my solution. It’s just not going to be used a a lot. Having separate hvac systems means if one goes down you have a backup.

Just some thoughts. What ever you do I really like the variable speed compressors. That’s the only requirement for my next system as it’s quiet and allows for zoning with a single compressor.

 

[peakbagger]

John Seigenthaler the hydronics heating guru has done a lot of articles on integrating air to water heat pumps with boilers. Hard to beat his suggestions (although they are complex.

 

[salecker]

Being your plan is a back up/shoulder season system why not run duct work for your AC using a gas or oil fired system?
The cost is going to be low compared to your radiant side.
Many will state use split systems but they don’t work in extreme cold. My proof is 6 miles south of me for a restaurant that froze up solid in below zero teens weather under 30 days ago.
You couldn’t pay me to use a glycol system in anything I own. I managed a single estate with 3 houses on glycol so I speak from direct experience.

Whats the bad experience with glycol?
Asking as it is required in the cold up here.

 

[Bad LP]

Whats the bad experience with glycol?
Asking as it is required in the cold up here.

It's corrosive at every fitting. It's expensive, needs yearly checks for the protection rating. If you need to change out a pump or work on a line you are not just going to dump all that money on the ground like water filled system.

It's cold here too. I have several thousand feet of PEX in this house with most of it being buried in concrete. If it freezes I'm screwed. Really screwed but no way is that crap going in my own house.

 

[peakbagger]

My solar hot water system has been running for 25 years on polypropylene glycol. All copper piping, no leaks.

 

[salecker]

Thats what i have in my underground lines and house,polypropylene glycol
I have a side stream filter on the glycol side which cleaned up all the residuals from building the system out of used and new components.
13 years with zero issues...

 

[brenndatomu]

Glycol is certainly corrosive...but can be managed.

Avoiding Corrosion in Liquid Cooling Systems

Choosing the best heat transfer fluids for your liquid cooling system

www.boydcorp.com

 

[salecker]

I am not a pro...
Whats the difference between polypropylene and ethylene glycol?
I was told to use the polypropylene specifically for my boiler system.

 

[peakbagger]

Ethylene Glycol is toxic especially to animals. Polypropylene glycol (PEG)is not toxic (although additive packages can make it somewhat toxic). Ethylene Glycol is generally cheaper. PEG is used as a food additive and anyone who has had the "pleasure" of having a colonoscopy downed a bunch of it for prep.

Note PEG and regular Ethylene Glycol turns acidic if overheated. When pumps fail on solar hot water systems and the fluid goes stagnant in a hot collector it can go acid quick. This is what usually happens on old solar system the controls break down, the pumps stop and the glycol goes acidic and corrodes the piping.

On the big heat exhaust heat recovery coils we use on CHP plants, we make sure they are designed to run dry and drain coils in the summer to keep the glycol from stagnating.

 

[sloeffle]

Can you use methanol or ethanol if you are worried about the loops in your house freezing ? That’s what my geo system has in it.

I realize that probably won’t work in the boiler itself.

 

[salecker]

I looked at my extra drums
50/50 Polypropylene heat transfer fluid was on the label.

 

[dregier]

John Seigenthaler the hydronics heating guru has done a lot of articles on integrating air to water heat pumps with boilers. Hard to beat his suggestions (although they are complex.

I've been watching his educational videos, reading his articles, and I've read much of his modern hydronic heating textbook. While I'm trying to incorporate his ideas, I still haven't found an example that quite fits my situation. I'd love it if he would just design it for me!

 

[dregier]

How many square feet in house garage and shop? First thought is if you want ac you are going to need ducts. Seems simpler to have an air to air ducted heatpump separate from the boiler. Attic ducting wouldn’t bother me as much since it’s not going to be used too much in the winter. Variable speed compressor with two or three zones. Or just a couple mini split units.

Now I totally get the radiant heat but how often do you envision switching back and forth? (Do the two systems need to be integrated?) I see the heatpump running until temps drop consistently below freezing. if you want your shop or garage heated why not a mini split? Will you want humidity control in the shop?

I don’t think AWHP tech in North American is ready for prime time. Biggest reason is knowledgeable installers And technicians. I do think you would use the heatpump more in an integrated system. For instance our weather the past week has been near freezing at night up to the 50s. Fare at night or early morning and heatpump in the afternoon. But that could easily be done with separate systems too.

Based on my wood heat and heatpump combo wood Handels 75% or more of my heating. The cheapest way to install the heatpump would be my solution. It’s just not going to be used a a lot. Having separate hvac systems means if one goes down you have a backup.

Just some thoughts. What ever you do I really like the variable speed compressors. That’s the only requirement for my next system as it’s quiet and allows for zoning with a single compressor.

4600 sqft house, 800 sqft garage, and 1600 sqft shop. I definitely will need to duct for cooling but part of my house includes a suite which will not be ducted with the main house (I don't think we're allowed to because of fire code regulations) so I have to rely on straight hydronic heating for that portion.

Thanks for the suggestion. My plan B is to do the systems separately exactly as you suggest. With separate systems, I think I'll still add a backup boiler to the hydronic side, either electric or propane. John Siegenthaler has a good example of using a single thermal mass by connecting a backup boiler to a large buffer tank and only cycling the boiler through the upper 1/4 of the tank. That way I can at least keep the system somewhat charged and the heat running with the back-up boiler if I'm not around for a few days to tend a fire. But, as you said, I probably won't use the heat pump very much and I think I'd have to sacrifice a bit of efficiency on the cooling side if I go to a heat pump so I may just opt for straight AC and heat with hydronic only.

I hear what you're saying about AWHPs not being ready for prime time in NA yet. I had to hunt but I have actually found a Canadian AWHP company with reasonably local support where I live in south eastern British Columbia (Arctic Heat Pumps). They have proposed a few design ideas but they are complicated, expensive, and include a lot of tanks and heat exchangers. I was hoping someone on here had a working system similar to what I'm looking for. Would be really nice to integrate into a single system but it definitely has to make financial sense at the end of the day.

 

[dregier]

Being your plan is a back up/shoulder season system why not run duct work for your AC using a gas or oil fired system?
The cost is going to be low compared to your radiant side.
Many will state use split systems but they don’t work in extreme cold. My proof is 6 miles south of me for a restaurant that froze up solid in below zero teens weather under 30 days ago.
You couldn’t pay me to use a glycol system in anything I own. I managed a single estate with 3 houses on glycol so I speak from direct experience.

Thanks for the suggestion. From all I've read about glycol (and from your experience) I'd prefer to avoid but I don't think I'll have much choice with my detached shop as I only plan to heat it when I'm using it. I figure one small portion of the overall design with a bit of glycol is a sacrifice I'm willing to make.

 

[Bad LP]

Thanks for the suggestion. From all I've read about glycol (and from your experience) I'd prefer to avoid but I don't think I'll have much choice with my detached shop as I only plan to heat it when I'm using it. I figure one small portion of the overall design with a bit of glycol is a sacrifice I'm willing to make.

I get it. Sometimes it’s unavoidable in many applications.

 

[hedge wood]

I run polypropylene glycol in my shop floor and in my lines to my house. If I didn't I would have been screwed both times I had welds failed on my Garn right in the middle of heating season. The last time it failed it was -34 F and I had to fire up my LP furnaces. I just put a electric milk house electric heater in the Garn barn and that kept the heat exchangers and pipes that had water in them from freezing and my floor and lines to the house were fine with the glycol. I worked around glycol for years in the industry world and never had huge issues with it. It does take some maintenance.