Bruno Posted March 19, 2021 Author Share Posted March 19, 2021 11 minutes ago, ProDave said: If the flow and return to the HW cylinder is "the same" then something is wrong, like the HW is already up to temperature and the system should have turned off. You have got a proper "heat pump" HW cylinder with a large area input coil? Currently I have a wood burner which circulates hot water (please see first post). When the water in the burner reaches 60° its circulating pump starts to run, feeding hot water to the DHW cylinder. When the DWH cylinder's thermostat reaches the setpoint, it diverts the hot water to the radiators. It's normal to have the temperature in the wood burner to keep increasing (a little of course) while the DHW is being heated as the circuit is small and little heat is radiated. In 30min the setpoint temperature is reached. But of course I don't know what's the return temperature. But it seems that most DHW cylinders have much more water than the amount you can use. Apparently mine hasn't, it only has a small hot water circuit through it (around 3m he said). What the salesman told me is that, because there is only a small amount of water in the DHW heating circuit, the return temperature would be pretty much the same and the HP can't handle that. Link to comment Share on other sites More sharing options...
ProDave Posted March 19, 2021 Share Posted March 19, 2021 1 minute ago, Bruno said: But it seems that most DHW cylinders have much more water than the amount you can use. Apparently mine hasn't, it only has a small hot water circuit through it (around 3m he said). What the salesman told me is that, because there is only a small amount of water in the DHW heating circuit, the return temperature would be pretty much the same and the HP can't handle that. The drop in temperature from flow to return occurs because the heat input coil has transferred the heat into the stored hot water. It is not a function of heat losses in the short or long flow and return pipes. An ASHP usually has a temperature probe inserted in a pocket in the HW tank. If this senses the tank is up to the set temperature the ASHP will stop heating it. Typically the flow temperature would be about 5 degrees above the tank temperature. I suspect your salesman might not be the best system designer. 1 Link to comment Share on other sites More sharing options...
Bruno Posted March 19, 2021 Author Share Posted March 19, 2021 (edited) 10 minutes ago, ProDave said: An ASHP usually has a temperature probe inserted in a pocket in the HW tank Yes, that's considered. But according to him, that would be used to trigger the start of the HP when there is a temperature drop, not to control it directly. And that would of course heat both the DHW and the radiators. 10 minutes ago, ProDave said: I suspect your salesman might not be the best system designer. I tend to think that too. But as I don't know the "guts" of how an HP works, I can't debate. The price is really attractive for this HP (70° outside temp, A+++), but I can't get them to install it as I want it to, after all, they're the experts... Edited March 19, 2021 by Bruno Link to comment Share on other sites More sharing options...
Bruno Posted March 19, 2021 Author Share Posted March 19, 2021 1 hour ago, Bruno said: - it makes no sense to use the heat pump to heat DHW without heating the radiators (eg summer) as the unit's COP will be 1, so it would be better to use an electric resistor. Q: does this make any sense? If there are 30 degrees outside and I want to heat the DHW to 55deg, is the COP worse? It goes against everything I read.. And what about this, what do you think? Link to comment Share on other sites More sharing options...
SteamyTea Posted March 20, 2021 Share Posted March 20, 2021 (edited) 9 hours ago, Bruno said: The price is really attractive for this HP (70° outside temp, A+++), but I can't get them to install it as I want it to, after all, they're the experts Walk away from them now, it is going to cost you. DHW and Space Heating are different things, at different times and at different temperatures. They should be separated. Edited March 20, 2021 by SteamyTea Link to comment Share on other sites More sharing options...
J1mbo Posted March 20, 2021 Share Posted March 20, 2021 Obviously the DHW will only ever reach the HP flow temperature so the HP in that config will surely have to work on a fixed flow temperature basis and that will cripple efficiency. Link to comment Share on other sites More sharing options...
ProDave Posted March 20, 2021 Share Posted March 20, 2021 11 hours ago, Bruno said: it makes no sense to use the heat pump to heat DHW without heating the radiators (eg summer) as the unit's COP will be 1, so it would be better to use an electric resistor. Q: does this make any sense? If there are 30 degrees outside and I want to heat the DHW to 55deg, is the COP worse? It goes against everything I read... What makes you think a heat pump heating DHW only in the summer will only have a COP of 1? Most of us use our heat pumps all year including DHW only in the summer. When the air is warmer it should run at a better COP than when doing the same in sub zero winter temperatures. Link to comment Share on other sites More sharing options...
Bruno Posted March 20, 2021 Author Share Posted March 20, 2021 3 hours ago, ProDave said: What makes you think a heat pump heating DHW only in the summer will only have a COP of 1? It wasn't me. It was the salesman who told me that, I was just quoting him. Link to comment Share on other sites More sharing options...
MikeSharp01 Posted March 20, 2021 Share Posted March 20, 2021 4 hours ago, ProDave said: sub zero Maybe @Bruno lives even further north than you do - summers in such places must be very hard to imply a COP like that! Link to comment Share on other sites More sharing options...
Bruno Posted March 20, 2021 Author Share Posted March 20, 2021 No, I live in Portugal, sub zero is really, really rare. Not more than 10 nights per year, for sure Link to comment Share on other sites More sharing options...
SteamyTea Posted March 20, 2021 Share Posted March 20, 2021 54 minutes ago, Bruno said: No, I live in Portugal, sub zero is really, really rare. Not more than 10 nights per year, for sure Similar to Cornwall then. Link to comment Share on other sites More sharing options...
ProDave Posted March 20, 2021 Share Posted March 20, 2021 1 hour ago, Bruno said: It wasn't me. It was the salesman who told me that, I was just quoting him. You need a different salesman. Link to comment Share on other sites More sharing options...
Bruno Posted March 20, 2021 Author Share Posted March 20, 2021 (edited) 1 hour ago, ProDave said: You need a different salesman. Indeed. I've been asking locally and the thing with COP 1 with high outside temperature seems to be utter BS. But the price is just good. Damn. What about the current DHW cylinder, do you think that it can be used with the HP? Looking at the aroTHERM documentation it always mentions their own uniTOWER cylinder... Edited March 20, 2021 by Bruno Link to comment Share on other sites More sharing options...
J1mbo Posted March 20, 2021 Share Posted March 20, 2021 Ask the Vaillant people who designs their systems. It may well be Vaillant themselves, which adds some peace of mind. Ensure the installer you chose has the appropriate Vaillant certifications to get the full 7 year warranty on the system. Although they have their own cylinders, any heat pump cylinder would do - the coil surface area is larger than standard cylinders. Typically 3m2 or more. If you're in the UK and looking at RHI, it's worth playing with the design flow temperatures (even if this means changing some radiators) as lower flow temp = higher COP from the MCS product performance tables = higher payments. 1 Link to comment Share on other sites More sharing options...
Bruno Posted March 21, 2021 Author Share Posted March 21, 2021 14 hours ago, J1mbo said: Although they have their own cylinders, any heat pump cylinder would do - the coil surface area is larger than standard cylinders. Typically 3m2 or more. Mine is probably not a "heat pump cylinder", I suppose it's designed for using wood burners. But you gave me a good idea, I'll contact the manufacturer and ask if they can provide the coil surface area ? 14 hours ago, J1mbo said: even if this means changing some radiators I'm also looking into that, thanks. I'll replace 2 radiators and distribute the sections in the remaining ones. Link to comment Share on other sites More sharing options...
Bruno Posted March 23, 2021 Author Share Posted March 23, 2021 I found the installation manual from Vaillant. I'm pretty sure I would be able to do this myself ? Link to comment Share on other sites More sharing options...
J1mbo Posted March 24, 2021 Share Posted March 24, 2021 (edited) Certification is required to work on the unvented side (even though the ASHP cannot actually boil the water, though the immersion clearly can) and the system needs glycol. The other issue is that the system only gets 7 years warranty if installed and serviced by an approved installer. Edited March 24, 2021 by J1mbo Link to comment Share on other sites More sharing options...
Bruno Posted March 24, 2021 Author Share Posted March 24, 2021 (edited) Hi @J1mbo again thank you for your reply. 1 hour ago, J1mbo said: The other issue is that the system only gets 7 years warranty if installed and serviced by an approved installer. I did contact Vaillant Portugal and they replied quite fast about this. They didn't use the word "approved", they said "with experience and certification for heat pumps installation" To be honest, what I'm missing the most is to be able to talk to someone who is not "interested". I got very few replies to my quotation requests, but every one of those was contraditory in solutions and ALL of them want to share the DWH and radiator heating in some way. Here's a short list of what I got: - 2 suppliers quoted a 9kW, other a 12kW and other a 15(!)kW heat pump. The first said that my requirements are perfectly fine with 9kW and that everything above it is too much, the last said that 15kW is ideal so that the heat pump has enough reserve for very cold days. - None of the quotations have a diverter (3-way) valve. One wants to connect the DWH cylinder in series with the radiators, 2 of them wants to connect a secondary circulation pump to the buffer tank so that 1 goes to the radiators and another goes to the DHW cylinder (their justification is that this way I can get DHW and heating at the same time, I then told them that 40-50º might not be enough for DHW and they said "sure it is"), the last I have absolutely no idea how the system would work but there is no 3-way valve in the quotation. - At least one of the quotations has no integrated DWH control, I would use a clock and a thermostat to control the respective circulation pump. I don't want this, are we in 1990? ? - One has no buffer tank, other has a wrong buffer tank (specifically the Vaillant aroTHERM mentions clearly a 40l buffer, they quoted 100l), other has an hydraulic separator, other has both (!). - Some cared about my request to integrate the wood boiler, others didn't. - I specifically indicated that the ASHP needs to be placed on an outside wall as I can't have it on the ground (I live in an appartment). I did not see any mention to wall supports on the quotations, plus the 15kW heat pump I was quoted weights 170kg ? What a mess ? Anyway I will update this if I have more news. Edited March 24, 2021 by Bruno Link to comment Share on other sites More sharing options...
TonyT Posted March 24, 2021 Share Posted March 24, 2021 I think you would be best giving a specification of what you want to installers to ensure you get what you want. that way you can compare quotes. DHW under 50 deg is fine for me! Link to comment Share on other sites More sharing options...
Bruno Posted March 24, 2021 Author Share Posted March 24, 2021 29 minutes ago, TonyT said: I think you would be best giving a specification of what you want to installers to ensure you get what you want. Yeah, that's what I will do next. It also helps that I've read and asked a lot (thank you all), so now I also know a bit better what I want. 42 minutes ago, TonyT said: DHW under 50 deg is fine for me! It depends, I was discussing that today with one of the suppliers. In summer it's totally fine, even more than enough. But in winter it isn't: - on one side, my wife and kid love to shower with water which makes their skin turn red ? - on the other side, often we want that very hot water because the house (specifically the bathroom) is a bit cold. I hope to improve that with better insulation and the ASHP running permanently, but anyway I want to be able to choose ? Link to comment Share on other sites More sharing options...
SteamyTea Posted March 24, 2021 Share Posted March 24, 2021 You are not going to shower in water any hotter than 42⁰C. Just make sure the effective volume stored is large enough. Link to comment Share on other sites More sharing options...
J1mbo Posted March 24, 2021 Share Posted March 24, 2021 (edited) If you go Vaillant then the weather and room temperature modulation will be key to getting the systems to perform - so go with their controls, exclusively. But this only reinforces the need for diverted control on DHW - it will back off the flow temperature as room temperature set point is reached. Right now I’m seeing 31*C flow to the rads for example. That’s just not going to work with DHW. Edited March 24, 2021 by J1mbo Link to comment Share on other sites More sharing options...
Bruno Posted March 24, 2021 Author Share Posted March 24, 2021 1 hour ago, SteamyTea said: You are not going to shower in water any hotter than 42⁰C. Just make sure the effective volume stored is large enough. The DWH cylinder is the same we have now, 150l which is enough for everyone to shower. If I believe the temperature indicator on the cylinder, the DHW is heated to 60°. At that temperature we definitely need to mix some cold water. But I will need to look into improve the insulation of the hot water circuit, for sure ? 42 minutes ago, J1mbo said: If you go Vaillant then the weather and room temperature compensation will be key to getting the systems to perform - so go with their controls, exclusively. I will. Thank you. Link to comment Share on other sites More sharing options...
joe90 Posted March 24, 2021 Share Posted March 24, 2021 1 hour ago, Bruno said: In summer it's totally fine, even more than enough. But in winter it isn't: Why?, I heat my DHW to 48’ and cannot hold my hand under the hot tap or shower at that temp (and I like it HOT). Why would winter make any difference, the COP might be slightly less (reliant on RH rather than temp) Link to comment Share on other sites More sharing options...
SteamyTea Posted March 24, 2021 Share Posted March 24, 2021 35 minutes ago, Bruno said: But I will need to look into improve the insulation of the hot water circuit, for sure I added insulation around my cylinder, saved a couple of kWh a day. Though if you spread the showering out over that day, the HP can recharge it. If you take the showers at about the same time, there is no need to recharge until a bit before it is needed again i.e. early morning. Teach everyone to take shorter showers and the problems go away. I got my lodger to go from 25 minute to 4 minutes, or 275 litres to 44 litres. She smelt the same. Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now