ᐅ Replacement of Oil Heating System with Heat Pump, Underfloor Heating, and Wood Boiler
Created on: 2 Jun 2022 13:21
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Ebedi22Hello, we are planning to renovate the basement of our house. Current situation:
The energy consultant’s renovation plan includes the following initial steps:
Further insulation measures are planned in later phases of the renovation, but these are currently postponed because they are too complex and therefore too expensive. Additionally, I am considering installing a controlled mechanical ventilation system (for indoor climate and humidity) as a DIY project, as well as adding a wood heating system for winter (possibly a wood gasifier or replacing the old inefficient fireplace in the living room with something more efficient). We practically have access to firewood for free (just need to cut and bring it in).
I have already gathered useful information on this topic here in the forum, but I still have some questions:
Thank you very much for your patience in reading this and for any answers!
- Two-story house (living area approx. 243m² (2,613 sq ft), built in 1977) with flat roof (roof was renewed and insulated a few years ago)
- Underfloor heating on the ground floor (from the beginning), radiators in the basement
- The basement is partially built into a slope; so far only one room is actively used as an office and heated during the week, the rest is unheated
- Heating and hot water production are oil-based (annual consumption approx. 2000 liters (530 gallons) per year; the energy consultant independently calculated a final energy consumption of 39,500 kWh/year and a primary energy demand of 191 kWh/(m²a) (61,720,000 BTU/(ft²a)))
- Almost all windows on the ground floor have been gradually replaced over recent years
The energy consultant’s renovation plan includes the following initial steps:
- Insulation of the basement exterior walls including new windows (possibly internal insulation on the wall facing the slope)
- Insulation of the basement floor and replacement of radiators with underfloor heating
- Replacement of the oil heating system with an air-to-water heat pump
- According to the energy consultant, this will result in a specific primary energy demand of 49 kWh/(m²a) (15,790,000 BTU/(ft²a)) and an expected final energy consumption of 9,700 kWh/year
Further insulation measures are planned in later phases of the renovation, but these are currently postponed because they are too complex and therefore too expensive. Additionally, I am considering installing a controlled mechanical ventilation system (for indoor climate and humidity) as a DIY project, as well as adding a wood heating system for winter (possibly a wood gasifier or replacing the old inefficient fireplace in the living room with something more efficient). We practically have access to firewood for free (just need to cut and bring it in).
I have already gathered useful information on this topic here in the forum, but I still have some questions:
- I often read that a heat pump combined with underfloor heating is best operated without a storage tank. The reasons seem plausible to me, but I wonder how this is actually implemented. My heating installers’ quotes all include a storage tank (probably required by each heat pump manufacturer). Since I’m not a professional and don’t want to disregard manufacturer specifications (in case of problems or warranty issues, I would be responsible), my question is: Are there heat pump manufacturers that do not require a storage tank? Or more generally, how do proponents of “no storage tank” operate their systems at home?
- One installer generously offered an 18 kW heat pump, another suggested 12 kW. Considering the current oil consumption of 2000 liters, should not something around 8 kW be sufficient, especially combined with the wood heating?
- The “old” underfloor heating on the ground floor has relatively wide pipe spacing (you can feel warm and cold spots). The new underfloor heating in the basement will of course have closer pipe spacing (better heat transfer → lower supply temperatures). The heated floor areas are roughly the same size. Could this cause issues, for example with pressure, flow rates, or the old underfloor heating unnecessarily raising the supply temperature? If yes, what would be suitable measures?
- Until the children are old enough to move to the basement, only the office there will need heating for the next few years. From an energy-saving perspective, is it sensible and feasible to hydraulically/thermally balance the heating system so that the basement (possibly excluding the office) remains cooler, and then carry out a new balancing once the children move downstairs (which could coincide with future energy measures on the ground floor)?
- Is there any risk to the heat pump if the old underfloor heating is connected directly (corrosion, materials no longer used today, etc.)?
- The bathroom on the ground floor has underfloor heating but is not warm enough when the children bathe. So far, we manage by turning on the towel radiator in advance, which is connected to the high-temperature heating circuit in the basement. However, that will be eliminated. Could the towel radiator be connected to the hot water cylinder instead (properly sized, possibly a combi boiler, since we plan to install a fresh water station or similar for domestic hot water anyway)? Is this reasonable or even allowed? This would have the advantage that the heat pump could run more efficiently for normal heating and only provide a short-term higher temperature level for the occasional children’s bath. If not, any other ideas?
- Which type of (log wood) heating system would be recommended in this case? A wood gasifier in the basement or a new stove in the living room? As I see it:
- Advantages of a wood gasifier: heat can be used for heating the whole house or for domestic hot water, possibly eligible for subsidies
- Advantages of a living room stove: cheaper upfront cost, works during power outages, offers the “coziness factor”
- Disadvantage of the living room stove: heat is only locally available and cannot be used for (less efficient) domestic hot water production combined with the heat pump — is it realistic to expect any noticeable (electricity) savings in winter, or would that be offset by still needing the heat pump for the rest of the house and the hot water, with only the living room being warmer?
Thank you very much for your patience in reading this and for any answers!
D
Deliverer6 Jun 2022 20:50That’s quite a lot of information and a complex project. But here are some thoughts:
1: If you can’t (or don’t want to) go for a tankless installation, the lesser evil is to use a small, roughly 50-liter (about 13 gallons) buffer tank in the return line. Most manufacturers allow this, and it doesn’t cause much harm. I simply installed it and agreed to retrofit if there were any disturbances. Nothing can really break.
And while you’re at it: you don’t want individual room controllers, a second pump group, or a hydraulic separator. Just spend some time learning about it, do balancing for about a year, and that’s it.
2: You need the correct heating load calculation for the house. Once the heat pump is installed, your heating behavior will change. You’ll heat all rooms continuously, not just parts of the house on demand. That’s both more efficient and needed because a system sized only for partial heating would have to be much larger. Once you have the heating load—ideally calculated room by room—choose a suitable heat pump WITHOUT extra safety or domestic hot water allowances. 18 kW is probably too big, but 8 kW might be tight if the 39,500 kWh/year figure is accurate.
Oh, and if that energy demand is right, you should improve the building’s energy efficiency beforehand. You’ll have money for that once we get to the last point.
3: Yes, it can be problematic. Solutions include adding extra radiators in certain rooms, installing surface heating panels on some walls or ceilings, or installing one or two air conditioning units to help. The worst solution would be to run a much higher flow temperature just for those rooms.
4: Heat everything continuously. Lowering the temperature by two, at most three degrees (Celsius) is no problem; more than that creates two issues: first, the flow temperature must be increased because energy is taken out of occupied rooms, which must be compensated, making it less efficient than heating everywhere. Second, large temperature differences combined with ventilation can cause mold problems.
5: Yes, metals can cause deposits. But if you know what materials are installed or take water samples, you can handle this by adding appropriate treatment agents.
6: My suggestion for the bathroom would be an infrared heater that only switches on when needed. Alternatively, you could add an additional ceiling heating system.
7: This is the most controversial point, but you asked for my opinion:
Skip wood. It costs money and space, creates dirt, is tiring, and harms the environment twice. First, through odor and fine dust emissions. Then, like oil, burning wood releases CO2 that cannot be reabsorbed quickly enough. By the time your burned trees have regrown, climate change will already be beyond control. The sustainability argument is a myth. True sustainability means leaving wood in the forest.
The chimney sweep alone costs about as much as heating with the heat pump for two months. So financially, you’ll never recoup that expense. Economically, it makes no sense.
Then there’s an unsightly chimney on the roof taking up space needed for important photovoltaic modules, while partially shading several of them.
Since, as described above, it will never get really cold again because you heat continuously, there’s no reason to light a fireplace.
If none of that bothers you, I’d recommend a wood stove in the living room. It can actually provide backup during power outages and is much cheaper. Integrating and controlling a second heat source into your heat pump system will not only disrupt your hydraulic balancing but also empty your wallet. It’s unnecessarily complex and much, much more expensive than any savings you could make even with free wood. By the way, for the same reason, people replace solar thermal systems with photovoltaics. LOTS of photovoltaics!
Keep it simple! And enjoy this exciting project! 🙂
1: If you can’t (or don’t want to) go for a tankless installation, the lesser evil is to use a small, roughly 50-liter (about 13 gallons) buffer tank in the return line. Most manufacturers allow this, and it doesn’t cause much harm. I simply installed it and agreed to retrofit if there were any disturbances. Nothing can really break.
And while you’re at it: you don’t want individual room controllers, a second pump group, or a hydraulic separator. Just spend some time learning about it, do balancing for about a year, and that’s it.
2: You need the correct heating load calculation for the house. Once the heat pump is installed, your heating behavior will change. You’ll heat all rooms continuously, not just parts of the house on demand. That’s both more efficient and needed because a system sized only for partial heating would have to be much larger. Once you have the heating load—ideally calculated room by room—choose a suitable heat pump WITHOUT extra safety or domestic hot water allowances. 18 kW is probably too big, but 8 kW might be tight if the 39,500 kWh/year figure is accurate.
Oh, and if that energy demand is right, you should improve the building’s energy efficiency beforehand. You’ll have money for that once we get to the last point.
3: Yes, it can be problematic. Solutions include adding extra radiators in certain rooms, installing surface heating panels on some walls or ceilings, or installing one or two air conditioning units to help. The worst solution would be to run a much higher flow temperature just for those rooms.
4: Heat everything continuously. Lowering the temperature by two, at most three degrees (Celsius) is no problem; more than that creates two issues: first, the flow temperature must be increased because energy is taken out of occupied rooms, which must be compensated, making it less efficient than heating everywhere. Second, large temperature differences combined with ventilation can cause mold problems.
5: Yes, metals can cause deposits. But if you know what materials are installed or take water samples, you can handle this by adding appropriate treatment agents.
6: My suggestion for the bathroom would be an infrared heater that only switches on when needed. Alternatively, you could add an additional ceiling heating system.
7: This is the most controversial point, but you asked for my opinion:
Skip wood. It costs money and space, creates dirt, is tiring, and harms the environment twice. First, through odor and fine dust emissions. Then, like oil, burning wood releases CO2 that cannot be reabsorbed quickly enough. By the time your burned trees have regrown, climate change will already be beyond control. The sustainability argument is a myth. True sustainability means leaving wood in the forest.
The chimney sweep alone costs about as much as heating with the heat pump for two months. So financially, you’ll never recoup that expense. Economically, it makes no sense.
Then there’s an unsightly chimney on the roof taking up space needed for important photovoltaic modules, while partially shading several of them.
Since, as described above, it will never get really cold again because you heat continuously, there’s no reason to light a fireplace.
If none of that bothers you, I’d recommend a wood stove in the living room. It can actually provide backup during power outages and is much cheaper. Integrating and controlling a second heat source into your heat pump system will not only disrupt your hydraulic balancing but also empty your wallet. It’s unnecessarily complex and much, much more expensive than any savings you could make even with free wood. By the way, for the same reason, people replace solar thermal systems with photovoltaics. LOTS of photovoltaics!
Keep it simple! And enjoy this exciting project! 🙂
Thank you very much for the detailed response; it really helped me a lot. I’ve been thinking about it over the past few days, especially the last point.
Would it be possible with this setup to also use the heat pump for cooling in the summer?
The 39,500 kWh/year is the calculated value for the current condition. With the measures currently planned, the energy consultant’s calculations estimate an expected final energy consumption of 9,700 kWh/year. However, the first figure seems a bit high to me (we currently use just under 2,000 liters (527 gallons) of heating oil annually, but only the ground floor is fully heated), and the second may be somewhat low (the insulation measures in the first step only apply to the basement).
The insulation of the ground floor will come later because it would make the entire project much more complex (the terrace needs to be addressed, two grapevines are growing on the house, there is a covered balcony/winter garden across almost the full width of the house, plus it’s a party wall property and the insulation would have to extend onto the neighbor’s property, and the roof also needs to be adapted accordingly).
Photovoltaics (currently just under 10 kWp) are already installed, but so are the chimneys 😉
I see that a water-based wood heating system doesn’t make sense (even though it would have been great for domestic hot water, where the heat pump is less efficient). However, my motivation to use a wood heater as well is not to heat permanently. Rather, it’s to be able to heat additionally when it’s really cold and the (air-source) heat pump is least efficient (which will be just a few days per year) and to have a backup system (hopefully never needed):
Of course, one could argue this is quite unlikely, but three years ago very few expected a pandemic, and a year ago few expected a war in Europe.
In addition, the heat pump runs only on pure (self-produced) green electricity during summer and partially in transitional seasons. In winter, it uses the existing electricity mix, which is currently becoming less green (the growth in renewable energy generation is currently hardly keeping up with the increase in electric consumption). 😉
Deliverer schrieb:
1: If you can’t (or don’t want to) go for a system without a buffer tank, the lesser evil would be a small ~50-liter (13-gallon) buffer tank in series (on the return line). Most manufacturers allow this, and it won’t cause much harm. I just did it and signed that I would retrofit one if any problems occur. It can’t really cause damage.
And while you’re at it: you don’t want individual room controllers, a second pump group, or a hydraulic separator. Just spend some time learning about it, balance the system for a year, and you’re done.
Would it be possible with this setup to also use the heat pump for cooling in the summer?
Deliverer schrieb:
18 kW is definitely too much, but 8 kW might be a bit tight if the figure of 39,500 kWh/year (annual) is correct.
Oh, and: if the energy amount is accurate, you should improve the building’s energy efficiency first. You’ll have the funds for that when I get to the last point.
The 39,500 kWh/year is the calculated value for the current condition. With the measures currently planned, the energy consultant’s calculations estimate an expected final energy consumption of 9,700 kWh/year. However, the first figure seems a bit high to me (we currently use just under 2,000 liters (527 gallons) of heating oil annually, but only the ground floor is fully heated), and the second may be somewhat low (the insulation measures in the first step only apply to the basement).
The insulation of the ground floor will come later because it would make the entire project much more complex (the terrace needs to be addressed, two grapevines are growing on the house, there is a covered balcony/winter garden across almost the full width of the house, plus it’s a party wall property and the insulation would have to extend onto the neighbor’s property, and the roof also needs to be adapted accordingly).
Deliverer schrieb:
Forget about wood heating. Firstly, it costs money and space, creates mess, is labor-intensive, and on top of that, it harms the environment twice as much. First through smell and fine dust, then it releases CO2 just like oil, which CAN’T be reabsorbed quickly enough. By the time the trees you burn have regrown, the climate change issue will have passed (for us). Sustainability is a myth here. Sustainable is leaving the wood in the forest.
Then, the chimney sweep alone costs as much as two months of heating with the heat pump. So financially, you will never recoup that. Economically, it’s nonsense.
Also, there will be an unsightly chimney on the roof taking up space needed for important photovoltaic (solar electric) modules, while partially shading five others.
Since, as described above, it will never get cold again because you continuously heat through, there is no reason to light a fireplace.
If none of this bothers you, I would tend to go for a wood-burning stove in the living room. It can actually serve as a backup during power outages and is much, much cheaper. Integrating and controlling a second heat generator into your heat pump system not only wrecks the hydraulics but also your budget. It’s unnecessarily complex and therefore much, much more expensive than you could ever save even with free wood. Incidentally, for the same reason, solar thermal is being phased out in heat pump systems and replaced by photovoltaics. LOTS of photovoltaics!
Keep it simple! And now have fun with the exciting project! 🙂
Photovoltaics (currently just under 10 kWp) are already installed, but so are the chimneys 😉
I see that a water-based wood heating system doesn’t make sense (even though it would have been great for domestic hot water, where the heat pump is less efficient). However, my motivation to use a wood heater as well is not to heat permanently. Rather, it’s to be able to heat additionally when it’s really cold and the (air-source) heat pump is least efficient (which will be just a few days per year) and to have a backup system (hopefully never needed):
- in case of extended power outages (not necessarily a full blackout, since that would probably mean bigger problems, but even a local storm can cause multi-day power failures)
- I feel better having the required heating energy for winter stored on-site, not dependent on the supplier’s discretion (as happened with gas prices last winter, something similar could happen with electricity prices).
Of course, one could argue this is quite unlikely, but three years ago very few expected a pandemic, and a year ago few expected a war in Europe.
In addition, the heat pump runs only on pure (self-produced) green electricity during summer and partially in transitional seasons. In winter, it uses the existing electricity mix, which is currently becoming less green (the growth in renewable energy generation is currently hardly keeping up with the increase in electric consumption). 😉
D
Deliverer9 Jun 2022 21:24Ebedi22 schrieb:
Would it also be possible to use the heat pump for cooling in the summer with this setup?Yes. However, water-based heat pumps cannot dehumidify, which is the most important factor in our summers, so it’s not very effective. If you want to be comfortable, you will need an air conditioning system.Ebedi22 schrieb:
... an expected final energy consumption of 9,700 kWh/a.That would really be excellent. Make sure the heat pump isn’t oversized. If the consumption really gets that low, even the smaller 5 kW units are probably the upper limit (just estimating here, you should calculate yourself). Especially if you also have a fireplace, the electric heating element in the heat pump, and maybe an air conditioner as backup.Ebedi22 schrieb:
- in case of prolonged power outages
- I feel more comfortable having the heating energy needed for winter stored on site
There’s nothing wrong with a bit of caution. And if you only burn wood occasionally for emergencies or a cozy atmosphere, that’s not a big environmental issue. In that case, a comfort fireplace makes sense. Hydronic (water-based) systems require flow, so you can’t use them as a backup during power outages. Also, from a financial perspective, it wouldn’t be reasonable. The small amount of hot water (around 50°C (122°F)), where the heat pump isn’t quite as efficient, you generate at midday with your own electricity, so it doesn’t have much impact.This will be a cool project. Read up a bit more on heat pump heating systems, it will save you a lot of money and trouble. Good luck!
Things are gradually taking shape, and now it’s time to determine the sizing of the heat pump, or rather, to decide at which renovation stage it should be optimally designed. Installing the heat pump will effectively upgrade the energy performance of the basement, while the ground floor remains unchanged for now, but is planned to be renovated as soon as there is more financial and time flexibility.
If the heat pump is sized for the first renovation phase, it will be oversized after renovating the upper floor. If it is sized for the complete renovation, it will initially be undersized.
This leads to considering the suggestion to include an air conditioning system:
and use it for heating during winter as a supplement to the heat pump. Of course, the sizing must then be correct, which raises the following questions:
If the heat pump is sized for the first renovation phase, it will be oversized after renovating the upper floor. If it is sized for the complete renovation, it will initially be undersized.
This leads to considering the suggestion to include an air conditioning system:
Deliverer schrieb:
Yes. Since water-based heat pumps cannot dehumidify, which is the most important aspect during our summers, it’s not very effective. If you want comfortable conditions, you’re likely to need air conditioning.
[...] maybe even have air conditioning as a backup.
and use it for heating during winter as a supplement to the heat pump. Of course, the sizing must then be correct, which raises the following questions:
- Not only should the heating capacity fit the house, but also the cooling capacity. If these two differ significantly, could it make sense to install two or three smaller units instead of one large system and use them only as needed?
- How can I size an air conditioning system based on the heating load? For heat pumps, this is done using a design chart that shows heating capacity depending on a certain supply temperature and outdoor temperature. For air conditioners, I have not been able to find such a chart; at best, a fixed heating capacity is given at a specific temperature combination (e.g., 4 kW at 7°C (45°F) outdoor and 21°C (70°F) desired indoor temperature). But that doesn’t help me for design conditions in the deep winter. How should I proceed here?
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