Fixing mistakes of the past, Thermal Store and Fireplace with water jacket
Location: South Poland, climate zone 6a
Hello all,
4 years ago we renovated a wooden \ brick house and did our best to make the house airtight (not tested) and as insulated as possible. I had a company in the UK perform a Sustainability Review and Thermal Modelling that you are welcome to read in full attached, but the key takeaway for this question is the heating requirements that were suggested…
* Wood Stove on the ground floor for room heating and hot water ~10kW
…connected to a Thermal Store.
For the last 4 years, we have had a 8kW (6.4kW water \ 1.6kW room) fireplace as our main heat source and topped up as needed with some electric heaters, mainly in the bathroom when washing the kids. A few notes on this experience…
Positives:
1. This has mostly worked well with heating needed for approx 5 months of the year Oct to March.
2. 6kW Immersion heater added to the Thermal Store that uses spare Solar energy to heat hot water allowed us to have hot water in summer and benefit from sunny days in winter
3. I have sensors added to the UFH, Thermal Store, and Fireplace that allow me to monitor and automate the heating systems and UFH with great flexibility (with Home Assistant)
Negatives:
A. With the store at around 50% 75c (167f) we have enough hot water to heat the upstairs, downstairs our concrete slab soaks up all this heat in around an hour, therefor we mostly have a cool floor downstairs (and lots of hassle from my partner)
B. No mixer was added for Direct Hot Water so the water depending on the Store temp can come out of a tap at around 70c+ which is very dangerous
C. I noticed overnight we lose around 10c from the store and I believe this is due to thermosyphon happening around the UFH heating feed and return pipe, I can see this at the UFH Manifold temps
D. Feed for UFH is taken from the thermal store top (75c) and then mixed down at the manifolds (40c) so a lot of unneeded heat is stored in the UFH feed pipe
E. When we have the Immersion heater active we get a thermosyphon happening in the fireplace feed and return pipes in reverse (hot water goes return > feed) that heats up the fireplace water jacket and activates the pump. We have to manually close a valve on this loop to stop this.
F. The Minimum return water temperature for the fireplace is 55c, but as this is taken from the bottom of the store it is normally around 26-31c
Attached are the schematics of the system to give an idea of the setup.
I would like to resolve the negatives above, I do not have high confidence in the installer and would like to have a well-informed understanding of how to improve these before I talk to anyone or may undertake some of the work myself, my suggestions are:
Improvements
Resolve A: As per the design documents add a 10kW Ground Source Heat pump, this should bring the bottom half of the Thermal Store up to around 45c, this is enough for the UFH in the winter.
Resolve B: Add a mixer for DHW at the store to bring tap water down to 50c
Resolve C: Add a solenoid valve at the store on the UFH feed pipe that is closed when not in use.
Resolve D: Add a mixer for UFH feed at the store to bring water down to 45c. I still want to take the water from the top and not for example the middle of the store so I can tap into all the hot water in the store, sometimes it is better to heat the house than have hot water for washing.
Resolve E: Add a solenoid valve at the store on the fireplace return pipe that is closed when not in use.
Resolve F: Move the fireplace feed pipe from the bottom to the middle of the store where the temperature will be around 45-50c when the Immersion heater is not in use.
Other points:
1. With the suggestions above I will have the Immersion heater and Fireplace managing the top of the store, both will not be in use at the same time and the Heat Pump will manage the bottom of the store for months when heating is needed.
2. For F I could also have a mixer valve added to take any available heat from the top of the store and mix it with the feed to the fireplace to help get closer to 55c. But keen to not add extra complexity
3. I still have a spare coil at the top of the store for another heating system if needed I can add later
You can see the proposed schematics with these suggestions also attached.
Im keen to get feedback on my suggestions and would love to hear from others who have similar setups their experience. Sorry for so much content but I’m keen to provide as much info as possible for those interested 🙂
Thank you in advance!
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Replies
You can't have a solenoid between the expansion tank and the Thermastore. The best location for the solenoid would be on the supply line to the stove after the pump. If the two floor heat pumps have check valves in them (or can be installed), you should not need the 2nd solenoid.
As shown in your current diagram, all house heat can only come from the stove or the resistance element, none of the heat from the heat pump would be used.
To fix it, you need to move the cold water feed of the mixing valve to the middle of the tank bellow the resistance heater. This way if the water at the middle location is hot enough, it will only use that for heat and only use the top of the tank if that runs out.
Air to water heat pumps are readily available in UK, I would not bother with GSHP. If you get split unit (not monoblock) you can connect it straight to the Thermastore instead of the coil which reduces delta T thus operating costs. Make sure to get a vapor injection cold climate unit.
I really hope you have insulation under the main floor slab. I know lot of slab on grade there is missing it. If you don't have any, you will still have issues, probably even using up the 10kW heat pump output, as you are mostly heating the soil under the house.
Hi Akos, thanks for your reply.
"hope you have insulation under the main floor slab" - for sure, 20cm EPS for the main slab (60% floor area) and 10cm for the rest.
"can't have a solenoid between the expansion tank and the Thermastore" - this makes sense, thank you.
"best location for the solenoid would be on the supply line to the stove after the pump" - interestingly in this location I do have a check valve, but this does not stop the flow of heat, I am assuming the movement of heat is not strong enough to "trigger" the check valve. Before the pump is the current manual valve we use to close this loop so makes sense why this was added here.
"As shown in your current diagram, all house heat can only come from the stove or the resistance element, none of the heat from the heat pump would be used." my assumption here is that the heat pump heated water would go to the top of the store, so for example, if the fireplace is not used and we have no sun, the heat pump would heat all water above it. But, I can see this logic may be misplaced and the heated water from the heat pump would stay in the bottom area of the thermal store where the heat pump coil is located?
"To fix it, you need to move the cold water feed of the mixing valve to the middle of the tank below the resistance heater" I may have misunderstood how the mixing valve works here, if I attach the cold feed to the mixing valve to the middle of the store would it not only use that if the hot feed is too hot, so it will use the top of the store as the primary source of heat still?
"Air to water heat pumps are readily available in UK, I would not bother with GSHP" same here, its the most popular option for installation and most installers will only do ASHP. Im very tempted to go with ASHP but also keen on GSHP to take advantage of running the system in reverse and having cooling in the summer from the pump.
" I do have a check valve, but this does not stop the flow of heat"
I've seen this happen as well. Usually spring check valves better than a swing check but can still leak when debris builds up on the seat. Solenoid is a guaranteed off.
" heat pump heated water would go to the top of the store"
Heat pumps usually produce colder water ~45C. With your stratified tank, if the top is heated by the element or stove to say 55C, no heat from the heat pump will make it there.
You need to pull the supply water for the floor heat from a location in your tank that is bellow the output temperature setpoint of the heat pump.
"f I attach the cold feed to the mixing valve to the middle of the store would it not only use that if the hot feed is too hot"
Say the middle is at 40C, top at 70C and you want 45C water. It would use about 85% 40C water and 15% 70C water. So most of the heat would be sourced from the heat pump.
If you set the mixing valve to 40C, than 100% would come from the heat pump. Only if the tank drops bellow 40C would any of the top hot water be used.
Air to water heat pumps cool no problem. The issue is you would have to do some extra plumbing to isolated the heat pump and the UFH from the tank during cooling season. The simplest is to use a set of 3 ways and connect the UFH directly to the heat pump supply/return. You'll also need separate controls to control the cooling, might be simplest to set the UFH pumps to ON and adjust cooling by changing the setpoint of the heat pump output water temp. Make sure to keep this water temp setpoint above indoor air dewpoint.
"Solenoid is a guaranteed off." yep, this is my thinking, I have read about many options to stop thermosyphon but Solenoid just feels like it will work
Thank you for your example with the mixing valve, so if I understand correctly the valve mixes up to the desired template (and not down as I had been thinking). So, for example if the full store was at 45C and I wanted 40C it would only take this from the cool input of the valve.
"The issue is you would have to do some extra plumbing to isolate the heat pump and the UFH from the tank during cooling season. " - I believe this would be the same with the GSHP as well as I would not wish to store any cold water, while I have been dreaming of a GSHP for the last 6 years, I think you and DCcontrarian have killed my final justification for it!
>"Air to water heat pumps are readily available in UK, I would not bother with GSHP" same here, its the most popular option for installation and most installers will only do ASHP. Im very tempted to go with ASHP but also keen on GSHP to take advantage of running the system in reverse and having cooling in the summer from the pump.
A second vote in favor of ASHP. Because GSHP's are a niche product innovation tends not to arrive in their design as quickly. The latest modulating ASHP's are so much more efficient that they end up using about the same energy as a GSHP.
The comment on cooling puzzles me. ASHP's can produce cooling. If you're using hydronics it's a question of having an emitter that can capture condensation, some sort of fan coil unit. That unit would be the same regardless of the type of heat pump.
Thank you for your feedback, we have a large field next to the house where I was planning to place the GSHP, but your feedback is very valid and more and I'm nudged towards a ASHP. As for ASHP for cooling I will read more into this and will discuss this with the local installers so thank you for the suggestion.
Reading the energy modeling, they say the house is 200 m2 and the heating load is around 20 kW.
For those used to American units, that's about 2000 square feet and 70,000 BTU/hr. This sounds way out of whack for a house built to modern standards.
And my eye was drawn to this section:
"We suggest it may be possible to include some more thermal mass in the first floor (The
ground floor has a solid floor so there is good thermal mass there, though more would be a
benefit). One option we suggest considering would be if some internal walls could be
blockwork instead of studwork. For example, on initial inspection and as an example to
consider, the walls either side of the landing could perhaps be blockwork? "
Long-time readers know my views on the subject, I'll just say I consider those to be the words of a charlatan.
Hi, I think the concern was in regards to how the wooden log side of the house would be insulated, from outside: 20cm wooden logs, air barrier, 20cm mineral wool, vapour barrier, and plasterboard. So there was not as much "mass" on the inside to retain heat, we used fermacell boards for the floor and walls that gave us some additional mass.
But agree that many things can be mass (what I believe your point is) and we have not noticed any issues.
Comment about this statement: " With the store at around 50% 75c (167f) we have enough hot water to heat the upstairs, downstairs our concrete slab soaks up all this heat in around an hour, therefor we mostly have a cool floor downstairs (and lots of hassle from my partner)"
It took me a long time to realize this, so even though it seems obvious in hindsight I assume it's not obvious: the heat transfer from a heated floor to the room is entirely determined by the temperature difference between the floor and the air in the room.
The report says heating load for the house is around 4kW at a typical winter temperature of 0C. Total area is 200 m2. I'm going to use US units because that's what I'm more comfortable, then I'm going to ask ChatGPT to translate into metric. That's 14,000 BTU/hour at 32F over 2000 square feet. I'm going to assume that 60% of the heating load is on the second floor and 40% on the first, so the heating load on the first floor is 5600 BTU/hour over 1000 square feet. That's 5.6 BTU/hour per square foot. The rule is that the heat output of a floor is 2.0 BTU/hour per square foot for every Fahrenheit degree, so that equals a temperature difference of 2.8F. So if your room is at 70F that would put the floor at 72.8F. I find a floor temperature below about 78F to be undetectably heated.
Note that the above calculation is independent of water temperature or flow rate or any of the other details. It also ignores any contribution to the heating of the ground floor from the wood stove.
ChatGPT's response:
"That's approximately 48,752.92 Watts at 0°C over 185.80 square meters. I'm going to assume that 60% of the heating load is on the second floor and 40% on the first, so the heating load on the first floor is approximately 32,501.95 Watts over 92.90 square meters. That's approximately 350.73 Watts per square meter. The rule is that the heat output of a floor is approximately 4.4 Watts per square meter for every Celsius degree, so that equals a temperature difference of 1.27°C. So if your room is at 21.11°C, that would put the floor at 22.38°C. I find a floor temperature below about 25.56°C to be undetectably heated."
ChatGPT got it backwards, 1kW=3400 BTU.
Hi, agree that the issue we have is that we do not heat the ground floor often so the slab is very cold, when we do it takes heat quickly and releases it slowly as we would expect.
The temp sensor for the slab is around 60cm to the closest UFH pipe, when this is at 19c we find that the full house is a comfortable temperature to wear a short-sleeved t-shirt in winter.
"I find a floor temperature below about 25.56°C to be undetectably heated" and that's fine for us, we do not wish to feel the floor is heated, just not cold. A heat pump keeping the floor at a steady temperature I feel should meet our needs.
If you have any suggestions on the proposed changes to the plumbing would be most welcome :)
It's not something changes to the plumbing is going to fix.
"The temp sensor for the slab is around 60cm to the closest UFH pipe, when this is at 19c we find that the full house is a comfortable temperature to wear a short-sleeved t-shirt in winter." This suggests to me that the floor is contributing zero heat to the room. If it's really at 19C and the room is at, say, 21C, the floor should actually be absorbing heat from the room. Something else is contributing enough heat to meet the heating needs of the room and even warm the slab somewhat. My guess would be the wood stove.
>"I find a floor temperature below about 25.56°C to be undetectably heated" and that's fine for us, we do not wish to feel the floor is heated, just not cold.
If the room is already t-shirt warm with the floor at 19C, making the floor warmer is only going to make the room warmer. If you have multiple loops of tubing in the floor you can turn some of them off, which will make part of the floor warmer and part cooler. That's the best you can do.
I think this sensor is a little misleading and the slab is warmer, especially closer to the UFH pipes. A better example would be the supply temperature to the manifold that is around 35-40c with a delatT of around 10c for the hour or so it's on. If I can maintain a supply of heated water from a heat pump to this slab I believe I can maintain a comfortable level of heating without the need of the fireplace, if needed I can adjust flows, stop the manifold pump or close off circuits as you suggested (we have 4 in the main room for example).
The fireplace most definitely contributes to the heating of the space, too much if we have it on constantly (coldest days we find two burns meet our needs), heating the slab with water heated from excess solar also has the same effect of making the house feel comfortable.
The house is reasonably comfortable most of the winter with just the people and activities you have in the house and firing the fireplace once a day if no sun, but it would be nice not to have a cold floor that you need to wear slippers on and hot water for heating available on demand so a heat pump and modestly heated slab I believe would meet our needs. (it's a right pain leaving the house for more than a few days in winter as it takes two days to get back to a comfortable temperature and we cannot remotely activate the fireplace)
I think you're getting closer to where you're eventually going to end up: run the heated floor all the time in the winter, only run the fireplace when it's so cold that the heated floor doesn't provide enough heat.
I want to repeat what I said earlier:
The heat transfer from a heated floor to the room is entirely determined by the temperature difference between the floor and the air in the room.
This is so important, I want to make sure you understand it. If you don't, re-read it until you're sure you do.
You should be thinking in terms of a target floor temperature. If you don't have a contactless thermometer, get one. The question you should be asking is, what floor temperature is comfortable to me? Both at the lower end and the higher end. Then you need to figure out how cold it has to be outside where if you run the floor as hot as is comfortable it doesn't put out enough heat. That's the point where you need to start using the fireplace. What you then want to do is figure out a control mechanism where as it gets warmer outside, the floor temperature drops so that the room doesn't overheat. If you have multiple loops you can also choose to heat a smaller amount of the floor, if that works for how you live.
Notice I'm not talking about water temperature or flow rates or zone valves or mixing valves or any of that stuff. That will all be important eventually, but it all exists in the service of floor temperature. When we get to the point where you can say, "I want the floor to be at 27C and it's only at 24C" then we can start talking about those things. But we're not there yet.