Posted by: @rob-of-york

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Does this set up mean that I should reject having a volumiser or a buffer in the heat pump system design, since the water volumes already in the radiators make them unnecessary, or is there some other valid purpose for having these present?

You should (almost) always reject having a buffer, by which I mean any kind of tank between the heat pump and emitters that is connected both to feed and return.

A volumiser (by which I mean a tank in either the feed (preferable) or the return (OK) and plumbed in such a position that it is not engaged when DHW is being heated), will do no harm, so if an installer wants to fit one and there is space, I wouldn't make a great song and dance about it.  It might just make defrost a bit quicker and/or smooth cooling during defrost, but possibly not a lot.

Re TRVs I had planned to use them in bedrooms but found that just balancing the rads on the LSVs worked just as well or better, and of course guarantees max volume for defrost.  I retain one in the guest bedroom as a courtesy. 

There is some evidence that traditional mechanical TRV heads in a mostly fairly well balanced system work almost like an automated balancing valve.  This interesting paper, which sadly has a title that is somewhat misleading in relation to what their data shows, illustrates mechanical TRVs behaving in an analog (ie not an on /off) fashion.  Smart TRVs behave in a digital way (unsurprisingly). 

In essence what they did was deliberately adjust the balancing of one or two radiators (in an otherwise perfectly balanced system) so they overheated, then applied TRVs which fixed it.  The claimed 'cost saving' is solely due to that, in other words the effect of applying the TRV was to restore the cost (and temperature) to that of a properly balanced system.  

I am personally edging towards the conclusion that the best way to use TRVs at present may be to replace the heads with decorators caps.  They then become a 'better lockshield', more easily adjustable than an LSV (better 'valve authority').  Some decorators caps even come with a nice +/- symbol and a couple of arrows to tell you which way to turn it to increase/decrease the flow.

If you are considering phasing the project, then I would investigate the VAT implications. A heat pump installation is zero rated for VAT, due to its environmental benefits.  This applies to the whole project - heat pump, pipework, radiators, hot water cylinder, accessories, labour…

If you have some additional radiators installed months or years after the original heat pump installation, I suspect this later project may be subject to 20% VAT.  Maybe someone else on here can confirm or correct this?

Posted by: @grahamf

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If you have some additional radiators installed months or years after the original heat pump installation, I suspect this later project may be subject to 20% VAT.  Maybe someone else on here can confirm or correct this?

If it's a separate contract then it would not be zero rated because in vat terms it's a separate 'supply.'

If it's the same contract then maybe it could be, in that case some deeper investigation needed.  However this is unlikely because bus is paid on conclusion and your installer isn't going to wait months or years for 7.5k.

Posted by @jamespa

If it's a separate contract then it would not be zero rated because in vat terms it's a separate 'supply.'

Thanks both, that is very useful information given my current plans to first replace all of the radiators (about £4000) and then try the system out as low temperature heating over a full heating season with the gas boiler still in place before committing to an ASHP install. Potentially that costs me an extra £667 in VAT, since it is certainly a separate contract.

@rob-of-york-york if you are planning to install a flow meter, I would suggest considering a heat meter instead.  This measures flow and return temperatures as well.  Combining this with the flow rate gives you the heat output from the boiler or heat pump.The Heat Pump Monitor web site can get you set up with a full independent monitoring system which includes a heat meter, electricity meter (or buy a Shelly) and a Raspberry Pi running their software.

https://heatpumpmonitor.org/

https://shop.openenergymonitor.com/level-3-heat-pump-monitoring-bundle-emonhp/

@rob-of-york I realise you feel the need to be cautious, but it is worth thinking carefully about what this careful two stage approach buys you.  There are two possible outcomes.  You will conclude that the heat pump:

1. Will provide enough heat at the lower flow temperature.
2. Won't provide enough heat at the lower flow temperature.

If it works well and you decide that a heat pump will be OK, then you just wasted £700 on VAT.  OK you might save some money by buying a smaller heat pump that will still do the job, but you can't be sure that your experiment will help that decision.

If you conclude that the heat pump won't do the job, then you just wasted £4,000 on radiators that you may not need with a gas boiler.

In the end, you just have to be brave and go for it.  Even if you take the two stage approach, you will still have to summon up the courage to go ahead with the ASHP in Stage 2.  

I too was nervous about installing an ASHP.  Lots of people told me that the project would fail.  I decided to design for a relatively low flow temperature (42C) to give myself room for manoeuvre.  I was also very careful to choose a good installer - one that Mars recommended.  We violently agreed not to use a buffer tank, go open loop and run it 24 hours per day on weather compensation.  I was still nervous, but I still had options to improve our insulation or some aspects of the system.

In practice, the system is running a SCOP of more than 4 and possibly closer to 5.  The house was lovely and warm (20C) all last winter. On the coldest night of the year at -5C outside, the flow temperature was still under 40C.

Of course, you must follow your instincts and go with the risk profile that you feel comfortable with, but make sure you are clear about which risks you are mitigating and which you are not.

Posted by: @grahamf

 if you are planning to install a flow meter, I would suggest considering a heat meter instead

Thanks for the link to the monitoring kit. However, it's quite expensive for the initial investigation that I'd like to do, i.e. find out what the flow rate and the flow and return temperatures are with the boiler running and the circulation pump on each of its 3 settings. I think a flow meter (about £60) and the thermal camera that I already have should suffice for that.

Posted by: @grahamf

it is worth thinking carefully about what this careful two stage approach buys you.

 

The radiators are 55-years old and in some cases showing signs of sludge on the thermal camera images. We plan to replace them in the not too distant future anyway irrespective of whether we go for an ASHP or not. Further, the sizing isn't significantly influenced by that decision either, since we are going for K2s (K3s ruled out as too wide) and keeping to the same lengths as the existing type 20s but extending the heights into wasted space under the windows. There is one new radiator added in the kitchen-breakfast room which is a little cold at present, and where we found the blanked off pipes to a missing radiator that was removed during changes to the kitchen that pre-date us moving in. These reasonable radiator changes should mean we can run a maximum radiator temperature of 37.5C with flow temperature of between 40C and 42.5C depending on what flow rate and DT the pipework cans support. As the gas boiler runs weather compensation that adjusts the flow temperature down to 30C, it seemed a useful idea to make a low temperature heating trial using the boiler to mimic a heat pump, before installing one.

The really useful point that you made earlier was about the VAT. Saving £670 on unnecessary tax changes the rationale about when to do the radiator upgrade. Without the difference in VAT, it made sense to do the radiator upgrade first and gain more information before making the final decision on an ASHP. With the difference in VAT, that decision clearly needs to be made before the radiator changes. 

I still think knowing something about flow rates and flow and return temperatures would be useful to address concerns about the pipework; however, I can do that with the existing radiators in place, with only the requirement to get a flow rate meter fitted.

Posted by: @rob-of-york

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I think a flow meter (about £60) and the thermal camera that I already have should suffice for that.

If you have a flowmeter and two temperature probes, then you can calculate power, so you have a thermal power meter. Won't accumulate kWh like a real heat meter but still useful.

Thermal camera is too inaccurate for this, but there are cheap digital thermometers with remote probes which you can tape to your pipes. Wrap in insulation and it's pretty accurate.

Posted by: @rob-of-york

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These reasonable radiator changes should mean we can run a maximum radiator temperature of 37.5C with flow temperature of between 40C and 42.5C depending on what flow rate and DT the pipework cans support.

You've spent ages deliberating over flow rates being potentially noisy, whilst running your preferred lower flow temperature.

But remember that your design criteria heat requirement will only occur for a few days per year (or even just a few hours at a time) and increasing the flow temperature to 50° or even 55° if required would transfer a lot of extra heat energy for the same flow rate. Obviously that would affect the COP for a bit. However you could find there's actually no need to go that high once you have the system running.

@singlespeed 

Thanks, however, I think there is more to it than flow temperatures, which is the main factor affecting heat pump efficiency.

Radiator temperature difference to room temperature is what determines their output. Radiator temperature is the average of flow and return temperatures. Hence a flow temperature of 50C or 55C has the potential to deliver far more emitter output than 40C or 45C.

The power that can be transferred from the heat pump however depends on the difference between flow and return temperatures, irrespective of what the flow temperature is. dT10 here transfers twice as much power as dT5 and half as much as dT20, which is typical for gas boilers.

Heat pump systems are typically designed for dT5, however, this needs 4x the flow rate as dT20 to transfer the same amount of power. Further some heat pumps effectively set the flow rate to be constant e.g. Vaillant Aerotherm 7kW defaults to 0.333l/s, and lets dT vary. This flow rate may well be a stretch for my pipework, hence wanting to figure this out pre-install. (It appears this heat pump can be set to 50% of that flow rate which should be fine).

Posted by: @singlespeed

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But remember that your design criteria heat requirement will only occur for a few days per year (or even just a few hours at a time) and increasing the flow temperature to 50° or even 55° if required would transfer a lot of extra heat energy for the same flow rate

That's not really the case.  Average surface temperature (to first order flow temperature) determines energy transfer from rad to room.  This needs to equal the room loss. 

DT will then be determined by that and flow rate.  All else being constant, inreasing FT will not reduce the required flow rate or DT.

@jamespa since going back to bed and reading again this morning, I have looked for the edit or delete options 🙁 

There shouldn't be a requirement for elevated flow temperatures, as the newly specified radiators can emit the required heat at a the planned 40° to 42.5° flow temperature with respective DT5 or DT7. Pushing higher DT across flow and return may work IF the heatpump allows it. But ultimately it's a compromise.

Rob is concerned about the cost and disruption for replacement of the 15mm secondaries. However would it be possible to extend the 22mm primaries to the mid point of the 15mm secondaries. Creating two 15mm secondary branches downstairs and two 15mm secondary branches upstairs... If a pair of surface mounted of pipes is OK, then it may be possible to only lift the upstairs hallway floor to a suitable point, then drop down and create a small access downstairs to connect onto the existing pipes.