muhrix

Thanks @SteamyTea! Did you write that report?

I don't think it would be, since the insulation is currently exposed on the attic floor and the roof is just lined. The DHW cylinder is located just below where the solar thermal plant is (in the attic) and where I believe the ASHP plant will go. We don't have a lot of space for that. I'll discuss what is feasible and practical with my installer today. Let's see what we end up with. Yes, I saw those and I liked the idea. I'm just not sure where the expansion vessel would end up given the extra height due to the buffer tank.

Would you mind pointing me to that thread, I'd like to read that (I can try and search for it too, just in case you have the link or remember to title of the thread).

Yes, I will have TRVs. I intend to have most of downstairs on “always open” since it’s all fairly open plan. The bedrooms will have smart TRVs, mainly to call for heat and maintain an even temperature (I won’t use them to switch off zones). Our installer told us we need a buffer tank, but we don’t know where to put it (they haven’t told us the size yet). They suggested the attic, but it sounds wasteful). Does it make sense to you? What are your thoughts?

Good point, thanks! I have an LPG tank with a very old gauge, so I think that’d be difficult, I think. I like your suggestion, though - thanks. I’ll see how I can estimate our usage. I did not know this was unusual - I thought that is how heat pumps work best. Keeping them running when needed, allowing radiators to be like warm emitting a steady and low amount of heat. Doesn’t that apply to radiators? I was not planning on having them off over night. I assume the pump will switch off once return and flow temperatures are too close, then switch back on once the temperature has gone down and the thermostat calls for heat. Would you consider your heat pump under- or over-sized? Or maybe just right? Our house is about 200 sq. metres, detach, fairly well insulated. Thank you all for sharing your views so far, much appreciated.

Oh that didn't work...

Hello, To keep it short, I am in the process of having a Mitsubishi Ecodan ASHP installed at my property. I had one installer quote for a 8.5kW model, while another installer quoted for a 14kW model. I don't have any information regarding the calculations from the first installer (who did a survey taking measurements, looking at insulation in the attic, etc.). The other installer provided a few design scenarios (as I requested). For regular design temperatures, the heat demand is 11.7kW, while for 21C throughout it would be 12.3kW and 22C throughout 13.4kW. I was told that a 10% factor was added on for intermittent heating, and then I was wondering (I do not plan to heat the house intermittently)... Removing 10% from those calculations, I'd get 10.53kW, 11.07kW and 12.06kW respectively. That means in two design temperature scenarios I could get a smaller unit (but not the warmest scenario). However, it's still way above 8.5kW. I have been reading a lot, trying to educate myself on the details regarding controls and heat demand. As you all know better than me, those heat loss values are for the coldest days in the winter (~5% of the year in Eastern England?). So most of the time I'm likely to need even less than the calculated heat demand. A smaller ASHP (11.2kW) would be: cheaper to buy and to run quieter to run potentially cycle less able to modulate to produce very little heat (for "drip feeding" into the house) compared to a bigger one (14kW) Am I right in my thinking here? Any improvements to insulation would reduce heat demand. The house is already fairly insulated, but there are some drafty doors that I plan to fix (not sure the fact they are drafty was taken into consideration). Other than that, perhaps changing all windows to triple glazing (but getting rid of LPG is higher priority). Are the SAP calculations normally on the conservative side w.r.t. heat loss? While it seems that 8.5kW is under-sized (and hence unsuitable), and 14kW slightly over-sized (depending on the design temperatures mentioned above), is 11.2kW just right? Or will the 11.2kW turn out to be a "false economy" and turn into a "nightmare"? Oh, for DHW, there will be a 250-300 litre cylinder heated by the ASHP. The house has 17 radiators which will be sized appropriately according to the design temperature of choice. Any help/advice would be greatly appreciated! Thank you. Note: these are the two installers I had constructive and positive discussions with. Quite a few just ignored me, others gave me quotes via email just by using my floor plan, another was extremely rude to us in their visit... overall, finding it very hard to navigate this "mine field".

On the topic of buffer tanks and whether or not they are necessary/recommended. Is there any scenario where a buffer tank is not a good idea? Lots of posts to sieve through on this topic; apologies if this has been answered elsewhere and I haven't come across that yet. In my case, I have a heat demand of 13.3 kW and will get a 14kW Mitsubishi Ecodan ASHP. It's a monobloc and I was initially told it didn't require a buffer tank (one of the reasons to choose this model since we don't have space). My installer is now suggesting a 50-litre buffer tank with an explanation that it would help with efficiency and reduce the amount of starts needed. While those are valid points, I believe I can achieve good efficiency (COP) and ensure flow temperature is such that the ASHP does not cycle too often. The downsides of a buffer tank (not a heat a heat store), IMHO and for my case, are: - One extra pump consuming electricity and making noise (which due to installation location, will be noticeable, particularly during the night) - The tank will invariably dissipate heat (and at the moment the only place I could have it is the attic, which would be a waste) - The thermostat and smart TRVs I plan to use would not be directly calling the ASHP for heat (flow/return temp. of radiators would not always match the ASHP) Any suggestions on where I can better inform myself on why and when buffer tanks are required, how to avoid them, or whether I should convince myself I have to have one?

I don't think there is anything wrong with Grant heat pumps (the solar thermal panels I have are from Grant). The Aerona3 SCOP look really impressive! I really couldn't justify the cost and disruption of installing radiators big enough to go lower than 55C (as much as I would've liked to). That said, according to their specs, I'd need to go for the 17kW model as at 55C the ASHP would deliver 10kW where I am, while the house demands ~13.4kW. Price-wise, I don't think they are too dissimilar from what I found online. I too plan to experiment and keep both flow temp and DHW temp as low as possible.

Thanks a lot!

Yes, it is a high temperature. It is actually the highest flow temperature permitted for me to still get the RHI grant. According to the MCS certified product directory, the SCOP at 55C is 3.26. If I were to reduce to 50C or 45C, the SCOP would be 3.52 or 3.77, respectively. For the RHI, I'd have to go down to at least 42C in order to get SCOP of 3.9 and increase the amount I'd get over 7 years. My rationale was: going down to 42C to get an extra £700 over 7 years would not pay off the cost of much larger radiators and the additional cost of replacing them (pipework, etc.). I believe that, once the system is up and running, I can experiment with the weather compensation curve and lower the flow whilst keeping the house comfortable to get a better SCOP. If I'm honest, the 14 kW also feels oversized to me, but the surveyor who did the heat loss calculation issued me a very thorough report, answered all of my (probably silly) questions and used a commercial software to input measurements, etc. The house is already fairly well insulated, not a lot more I can do about that.

Thanks a lot everyone! Catching up with the helpful comments and suggestions after a long working day. More context of my use case: we just moved to our new house (refurb. 1970s). It's a 4-bed house and we are 2 adults + 2 kids. We will get a 14kW Mitsubishi Ecodan ASHP running at flow temperature of 55C. I'm already having to change 11 radiators for much bigger sizes and cost-wise (and radiator size-wise) I just couldn't go with a lower temperature even though I would have liked to. The house already had thermal solar panels and we really enjoyed the free hot water between July (when we moved in) and September. Well, I currently have a 250-litre cylinder and it seemed more than enough. I'd expect to heat the tank up to 50C with the ASHP, but you make a good point. I also checked information online and given the property size and number of occupants, a cylinder with 250-300 litres should be enough. I don't think I do. My installer was going to confirm, but said if I end up needing one, it would be a small 50-litre tank. I'm going for an unvented cylinder. I am after data. Not only from the cylinder, but from the whole system. Ecodan's Melcloud and the MMSP will give me lots of data. It's like a hobby. I enjoy DIY/electronics/software and home automation... If I can improve the overall system performance by 5% and have fun, it's a win-win. Sure, there is a limit; the time and budget at my disposal as well as considerations such as those that @Gus Potter highlighted (I'd still like a system that works if I were to sell the house as he said). @markocosic Unfortunately I don't have PV (there were 2 solar thermal panels already installed). I am trying to make sure I can take advantage of PV, but it's not something I'll afford in the foreseeable future. I am going to check the Newark and World Heat cylinders as suggested. @Iceverge would you happen to have a reference or more context regarding the equation you wrote down? I'm curious to try and understand the theory/rationale. One interesting point I picked up is how people normally heat their cylinders (it's the first time I have one). It seems that one method is to heat to temperature and keep topping up given some hysteresis. The other method I came across was setting the timer to heat the water at given times and manually turn to top up when needed. A third alternative approach is to do what Mixergy proposes to do (estimate demand) and heat accordingly. Is there another method? Depending on the method, I can see it would have an influence on the most appropriate volume. Thanks again, everyone! I'm really enjoying the discussion!

Thanks for sharing your sister's use case, Gus. You make some very good points in your comments. I appreciate your advice.

Thanks for the suggestion! I came across that model, but I couldn't find any more technical details regarding dedicated solar volume or energy rating (kW) of coils (surface area is a little subjective). I'll try and contact Telford and ask for it.

Hello, I am in the process of replacing the LPG tank in my property with an ASHP (Mitsubishi Ecodan 14kW). I currently have 2 solar thermal panels which cover all hot water demand for 5-6 months in a year (and I'm planning to keep them). It has been really difficult for me to find a suitable DHW cylinder that (in my limited understanding) would work well with ASHPs and solar thermal panels. Even harder to find options where I'd be able to monitor water temperature at different heights. Ah - the cylinder volume should be 250 litres. I really like the idea of Mixergy tanks because of volumetric heating, the plate heat exchanger (more efficient), the diffusers (for better stratification) and all the data I can get from the tank. However, they are very expensive and, for the ASHP+solar thermal versions, I cannot do volumetric heating (I've been told so by Mixergy). More annoyingly, it seems there is no dedicated solar volume, so it'd be hard to make the most of the already installed solar thermal panels if I want hot water in the morning. The pre-plumbed Mitsubishi cylinder ticked a lot of boxes, but it has 3 pumps and my installer warned me of audible noise (the cylinder is to be installed in a bedroom cupboard). It's also very expensive. I looked at options from Gledhill, Joule, Kingspan, Heatrae Sadia, ... and neither of them looked perfect. Yes, I am likely overthinking here, but would anyone have any advice or experience they could share with me? I haven't ruled out any options yet. Thanks a lot!