I got the pipes on the hot water cylinder lagged (although ran out for a small area). Shocked at the difference it made. Heat loss went from 0.46 degrees per hour to 0.38 degrees per hour. Also got the water onto the schedule to only heat 1 hour in the morning which was lovely and hot this morning. I'll get some more lagging next week to do the last few pipes that are exposed.

Weather was slightly warmer last night (+1 to +6 degrees vs -4 to +4), but only used 33kw yesterday (COP 2.7) vs the 41-74 that we've been using the last week or two. Will keep tracking and see if it stays more sensible.

Posted by: @catapult

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@iancalderbank @derek-m I got some thermometers onto the pipes to/from the buffer. At flow temp of 37 degrees

from ASHP to buffer 36.7

from buffer to rads 34.8

from rads to buffer 33.4

from buffer to ASHP 33.8

ok so lots to consider here. first thought: is this definitely a radiator system? if so you've got some quite low temps in the rads (implied MeanWT of 34.1), so you're at least doing well from being able to run the ASHP at the lower temp end of its performance curve for a rad system. Or is it UFH (I thought I saw something a bit like an UFH manifold in one of the pictures)?

secondly the DT between flow and return , buffer to/from rads seems to be really low @ 1.4C. heat transfer is proportional to DT times flow rate. heat pump rule of thumb DT is 5C. 1.4C implies either an enormous flow rate or you're just not getting much heat through to rads. DT on the ASHP side is only 2.9 as well. did you say you had a flow rate number for the ASHP side from the heat pump itself? worth monitoring the temps on inputs and outputs of the manifolds as well to see what they are doing.. is it the same as the rad-side buffer pipes, or there some other mixing going on (you had a complex pipe topology I recall).

there might be a few tweaks that could come out of this, but (In my opinion) this will probably conclude in substantial physical plumbing changes being required to achieve a system that is efficient, vs one that does work but is inefficient. please read brendon's article that I posted link to if you've not done so already.

 I think the basic question you need to ask yourself at this point is this:

Are you from the side of the spectrum that is keen to do this yourself, with free no-warranty guidance from forum members, make mistakes, likely have to break it whilst you do so (so one for the summer!) learning along the way etc?  (hope this doesn't come over as rude - certainly not intended that way). or do you want to pay a pro to sort it? 

Posted by: @iancalderbank

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this will probably conclude in substantial physical plumbing changes being required

I recall from the initial post that this system has individual flow and return to every radiator (hence the manifolds), or almost every radiator (see first couple of posts).

That may allow for a less dramatic change (probably focused on radiators, and the buffer tank).  The MCS standards require the design meets 99% days of the year heating needs at design temperature.  If you are prepared to go a bit lower than 99% and with slightly lower design temperatures there will likely be more, and less disruptive options with the current setup.  e.g. adding fans to the radiators, sizing up some radiators or both to some.

Posted by: @catapult

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@iancalderbank @derek-m I got some thermometers onto the pipes to/from the buffer. At flow temp of 37 degrees

from ASHP to buffer 36.7

from buffer to rads 34.8

from rads to buffer 33.4

from buffer to ASHP 33.8

A buffer tank can be a problem with ASHP systems, since it makes balancing the flows and temperatures around the system more difficult. Probably the easiest way to modify, and hopefully improve your system, would be to connect the pipe from your heat pump directly to the pipe to the radiators, bypassing the buffer tank in the process. Then connect the return pipe from the radiators to one of the upper connections of the buffer tank, and plug the remaining open connections. This would convert your buffer tank into a volumiser, so that the water going to the radiators is now at full LWT without any possible mixing to cool it down. Once done it may also make the two additional water pumps surplus to requirements.

To carry out this modification would necessitate draining the system, so in the meantime you could try varying pump speeds to try to balance the flow rates and even up the temperatures.

The objective is to get the water going into the radiators as close as possible in temperature to the LWT of the heat pump.

@catapult Is this a diverter ?  If so, I would be interested in which one, and where you got it.

@william1066 yep agree. tbh what I was going through was exactly what derek-m just suggested. drain down and rebuild without buffer is what I'd do (But not sure'd I'd do in january). wasn't clear to me whether the OP is the type to want to do this level of modification themselves.

Posted by: @iancalderbank

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@william1066 yep agree. tbh what I was going through was exactly what derek-m just suggested. drain down and rebuild without buffer is what I'd do (But not sure'd I'd do in january). wasn't clear to me whether the OP is the type to want to do this level of modification themselves.

Actually looking at the photo's, if there are isolation valves in appropriate locations, it may be possible to shutdown the system, and isolate the buffer tank. Then remove the pipework to the air vent on the top of the buffer tank, and using a suitable hose siphon most, if not all, of the water from the buffer tank into suitable containers if it contains antifreeze.

Re-pipe the system as previously suggested, then refill the buffer tank, which is now a volumiser, either with the collected antifreeze mixture or just cold water. Open the isolation valves and get any remaining air out of the 'volumiser' and re-pressurise, then restart the system.

It may also be necessary to adjust the WC curve to accommodate the anticipated improvement in efficiency.