Hi Mars,

I have seen differing opinions on buffer tanks, some say always have one and other say they are just a bodge for a badly designed system. 

If they are for them then advice on sizing and their preferred way to plumb them.

I would be in interested in their opinion as I do get a lot of short cycling.

Thanks. 

Adam

@adam-c If your system is short cycling, then you do not hve enough open zone system volume, you sould have between 10 and 20 liters, per Kw output open zone (witout any control) to prevent cycling and low flow errors. For best performance, create an open zone system, remove third party controll and allow the heat pump to control the house temperature, if you still have short cycling, you need to increase the system volume by installing a tank on the flow pipes only.

If you are preaired to loose 20-30 of your efficiency, you can install a buffer tank using the scizing above. The buffer tank will de-couple your heat pump fron your heating system but you will need to control your system. This means that the heat pump just heats the buffer and the heating system draws off that. Not a bad idea with a boiler, as unlike heatpumps, a raise in temperature has little effect on the performance. Please be aware, we have seen an up to 5 times higher heating bills using this methode. Most complaints about high running costs are due to buffer tanks.

@heacol,

Brendan,

A couple of questions. What would you consider to be short cycling as opposed to acceptable? Also, is installing a tank on the flow pipes different to a buffer tank?  

Kev

From me:

What is the best way to integrate a zone controlled house (Evohome) with an ASHP?  We currently have a buffer tank system with each room/zone able to call for heat.  Evohome is set for minimum cycle time of five mins and set to Honeywell's heat pump suggested settings. (I did all this the installers bodged it and ran but that is another epistle.) As there are just two of us in a biggish house I don't want to go to the "just let the heat pump heat the whole place" system it seems wasteful.

@kev-m It should not be cycling at all theoretically, however Mitsubushies are not renowned for their performance and this may be one of the issues, What may be happening is the flow is too low and as the units operate onflow temperature and not Delta T, it tries to get it up to temperature as quickly as possible, it then has a minimum run time for oil control, by this point the temperature has overshot, and it shuts down, waits for the flow temperature to drop, and repeat. It is not having enough heat taken away initially to allow it to start modulating.

Yes, a tank installed on the flow pipe only increases the system volume of the system, allowing the heat pump to run longer, store the high-temperature heat allowing it to be used when the unit turns off. A buffer tank has 2 circuits, one for the heat pump and 1 for the heating system, It does not store any heat but blends down the temperature resulting in loss of performance.

@heacol I'm probably confusing heat pump running with central heating pump running.  I have no idea and no way of seeing when/if the heat pump is running but can hear the central heating pump in garage behind my study wall. (16kW Vaillant)

@witchcraft It is very difficult, if not impossible, to zone a property and get performance unless your heat delivery system in the area you are running is significantly larger (2-3 times) than it should be as the areas you are not heating steals heat from the areas you are. You are also likely to have cold periods and have an uncomfortable house.

Question from me:

What are the risks of oversizing an ASHP, and are there ways they can be mitigated? For example, might minimum flow rate be a problem (if most TRVs off, for example) and would a buffer tank help?  Is oversizing primarily a problem with fixed-speed ASHPs which have to modulate by cycling on an off, rather than inverter ones that can run more slowly? How low can a typical inverter ASHP efficiently modulate down its nameplate capacity?  

@Prunus, I don't know about flow and buffer tanks but I think the minimum output depends on the same conditions that affect the COP.  If the COP is high, so will the minimum output be.  Page 62 in this link has the outputs/COPs in different conditions for Mitsubishi ASHPs.  I'm sure you can get similar data for others.

LIBRARY.MITSUBISHIELECTRIC.CO.UK "https://library.mitsubishielectric.co.uk/pdf/book/ATW_Databook_R32_2020#page-60-61"

Ecodan ATW Databook R32 2020 - Document Library - Mitsubishi Electric

My 14kW Ecodan shows a minimum output of 5.2kW at 12C and flow 35C; the closest to the weather we have now.  The 11.2kW Ecodan is 4kW minimum in the same conditions.  At the moment, my ASHP is cycling every 30-40 minutes, I presume because its minimum would heat the water more than the weather compensation curver says it should. Maybe the smaller ASHP would run constantly?  Mine seems to when it's a little bit colder. However, it's very mild atm and when it gets much colder, my ASHP should be working less hard than a smaller one and COP should be better. It's a bit of a balancing act I think.

For various reasons I don't know what my COP is atm but I do know the minimum power consumed is about 1.1kW.  If the minimum output is 5.2kW, that puts my COP at nearly 5, which sounds feasible, so it sort of all adds up. 

Of course, the bigger ASHP is also more expensive.

@prunus, thanks for the questions above. There are some really good ones there.

Posted by: @kev-m

At the moment, my ASHP is cycling every 30-40 minutes, I presume because its minimum would heat the water more than the weather compensation curver says it should. Maybe the smaller ASHP would run constantly?  Mine seems to when it's a little bit colder. However, it's very mild atm and when it gets much colder, my ASHP should be working less hard than a smaller one and COP should be better. It's a bit of a balancing act I think.

Hi @kev-m (and maybe @derek-m will have a view on this too) I'm now bumping into a similar problem. Weather compensation seems to mean my heatpump is trying to operate below its minimum on warmer days (3.1kW @ 15°c ambient, 35°c water, for example) so it cycles often, which seems bad for CoP.

I know it's against the orthodox view, but I'm wondering if it could be worth raising the high-outdoor-temperature end of the heat curve, so deliberately warming the house with relatively cheap heat when it's above 13°c or so out (instead of aiming to hold a constant temperature as the curve currently does), but telling the smart thermostat to impose a maximum temperature in those conditions and let it switch the heating off for an hour (or more) when it gets too warm. In a way, it's still cycling, but with a much longer period, possibly even only one on-off-on cycle per afternoon.

Then as everything cools in the evening, the thermostat can go back to its high maximum and get out of the way, weather compensation takes over again, the heat pump is running above minimum and all is good like before.