ASHP retrofitted into thermal store with solar thermal and biomass stove

[RodW]

Hi,

 

I posted the following a couple of weeks ago to the new users thread but I didn't get any responses.  I'm trying to help an acquaintance, on an unpaid basis,  to minimise the amount of relatively new equipment that gets discarded.  If we are being unrealistic trying to retain so much of the existing system that would be useful to know.  The installation is near Braintree, Essex. 

 

Many thanks,

 

Rod

 

Quote

Hi,

 

I work on ventilation systems and thermal modelling for PH buildings and I've stayed in touch with one of my clients from 2011.  He specified the heating system that uses a large thermal store, Akvaterm 1000l, supplied with heat by a biomass stove and solar thermal.  The DHW and space heating are supplied from the TS.  Space healing is via UFH on GF and duct heaters in the MVHR supply air to the FF. There is a PV system on the roof but this is not linked to an immersion in the TS.  The original heating contractor is no longer involved.

 

As he has become older the level of physical work required to operate the stove every day through the winter is too high.  He wants to retain the ability to use the biomass stove for special occasions/resiliency but adapt the heating system to include an ASHP.  He has been contacting ASHP installers but none of them are interested in integrating with the existing TS.  We know this will not be the best way to run an ASHP but the space heating demand of the building is very low. 

 

All the ASHP installers I know design for compliance with RHI so this is too left field for them.  I thought I would ask here to see if retrofitting is a viable option or whether there is a better approach.

 

TIA

 

Rod

 

 

[joth]

Just a thought - you might get some more responses if you make a straw-man proposal or ask a more specific design question.

 

You mention the goal of adapting the heating system to use an ASHP. Is that space-heating only, or DHW too?

My initial thought would be to split the UFH from the legacy system and drive that exclusively from the ASHP. If the heating demand is low, that (plus the heat loss from the TS, assuming it's in the thermal envelope) maybe sufficient space heating for day to day. MVHR post heaters and towel rads could get the same treatment, depending how much they're actually doing / needed.

 

Heating a one tonne TS from an ASHP sounds a miserable task. If the solar thermal doesn't heat it enough in winter and the stove rarely used, adding an on-demand electric boiler to the output of it would allow it to ride through the darkest months and still have the use of those parts for DHW when the stove or sun is available.

 

 

[RodW]

The space heating demand is less than the DHW.  The solar thermal covers about half of the DHW i was hoping an ASHP could do space heating and the remaining DHW.  The UFH is only to part of the GF and the duct heaters do make the FF bedrooms comfortable.  I hoped that UFH and duct heaters would be linked direct from the ASHP,.  They require different water temperature but possibly flow temps to the duct heater could be altered depending on the weather, to run longer at lower flow temperatures. 

 

The thermal store needs to stay to allow the biomass stove/boiler to be used,  My original thoughts were: I didn't know if it is practical to get heat from the ASHP into the thermal store, it has 3 coils, 2 for solar thermal and 1 for the DHW.  Water is circulated from the TS through the biomass stove.  Maybe a plate heat exchange between the ASHP and the TS, using the ports no longer used by the space heating equipment, but that is another lossy step with additional hardware that will need maintenance.   Not linking the ASHP to the TS avoids this. 

 

Are there conditions where it would make sense to combine the TS with a separate water tank heated by the ASHP? 

 

The on demand electric boiler idea is interesting for the DHW.  If we adopt that route I am not sure if an ASHP is justified just of the space heating.

[joth]

1 hour ago, RodW said:

The thermal store needs to stay to allow the biomass stove/boiler to be used,  My original thoughts were: I didn't know if it is practical to get heat from the ASHP into the thermal store, it has 3 coils, 2 for solar thermal and 1 for the DHW.  Water is circulated from the TS through the biomass stove.  Maybe a plate heat exchange between the ASHP and the TS, using the ports no longer used by the space heating equipment, but that is another lossy step with additional hardware that will need maintenance.   Not linking the ASHP to the TS avoids this. 

 

 

A simpler option is to have a new small unvented cylinder heated from the ASHP and take the UVC input from the TS. So if the TS is hot (stove or solar active) the ASHP won't have to work, but if they're not it does. (i.e. same idea as the on demand  electric heater but ASHP friendly). It's still a lot of systems to maintain, but you _may_ be able to get the MCS installer to do this (basically tell the to take the UVC "cold in" feed from the TS and otherwise it looks like a bog standard install)

 

is the gargantuan TS specified to enable multi-day storage e.g. to cover less sunny days,  or because the daily demand is very high (running a guest house)? And what is the peak demand (5 showers running at once?). This sort of thing is probably influences the choices a lot.

 

 

[RodW]

Ok, I'll see if there are issues with a adding a second tank heated by the ASHP.  The plant room is on the first floor but the structure is already sufficient for the existing tank.

 

55 minutes ago, joth said:

 

is the gargantuan TS specified to enable multi-day storage e.g. to cover less sunny days,  or because the daily demand is very high (running a guest house)? And what is the peak demand (5 showers running at once?). This sort of thing is probably influences the choices a lot.

 

I wasn't involved in the TS sizing.  It may have been sized to provide space heating and DHW with only a single biomass burn per day. The actual DHW usage is quite modest.  

[arg]

I suspect that a major factor in sizing the TS was to make sure there’s room to absorb the heat from a run of the biomass.  As I understand those things, you fill it with fuel and fire it up - you then get a huge load of heat out of it over the next few hours, with no way to stop it if you’ve overdone the fuel.

[RodW]

On 15/07/2021 at 20:15, arg said:

I suspect that a major factor in sizing the TS was to make sure there’s room to absorb the heat from a run of the biomass.  As I understand those things, you fill it with fuel and fire it up - you then get a huge load of heat out of it over the next few hours, with no way to stop it if you’ve overdone the fuel.

 

Thanks, that is a good point.

[newhome]

I have a TS and enquired about replacing my boiler with an ASHP. 2 MCS installers said that the TS would need to be replaced with an UVC as the TS temperature was too high for the size of ASHP that was appropriate for my home to provide DHW at the temperature needed and the COP at a decent level. The 3rd one quoted for retaining the TS but the ASHP quoted was big and expensive and the total quote was circa 30k. 
 

 

[Temp]

How hot does the TS need to be to heat your house? Can you just run it cooler? 

 

For example DHW from a shower should be less than 48C. So a TS temperature of 45 to 50C should be just about ok for DHW. Is that too low to heat the house or too high for ASHP?

[Nickfromwales]

On 14/07/2021 at 17:32, joth said:

A simpler option is to have a new small unvented cylinder heated from the ASHP and take the UVC input from the TS. So if the TS is hot (stove or solar active) the ASHP won't have to work, but if they're not it does. (i.e. same idea as the on demand  electric heater but ASHP friendly). It's still a lot of systems to maintain, but you _may_ be able to get the MCS installer to do this (basically tell the to take the UVC "cold in" feed from the TS and otherwise it looks like a bog standard install)

Yup. 100% spot on solution, and the only sensible one for this situation.

You could fit a large split ASHP ( high temp ) here, and then get up around 80oC, but it would be bulky and very noisy.  Heating would simply be toggled in / out between the TS or the ASHP  by a diverter valve which changes state with the temp of the store. Pretty easy to achieve.

[RodW]

 

23 hours ago, Nickfromwales said:

Heating would simply be toggled in / out between the TS or the ASHP  by a diverter valve which changes state with the temp of the store. Pretty easy to achieve.

 

That is an interesting point.  I assumed we would just move the space heating over to the ASHP but this would allow the heating to work with the TS if its hot enough. There would a link to the ASHP controller calling for heat when there is demand in the building and the TS is too cold.  Would there be a volumiser attached to the ASHP that the diverter valves connect to?

[Nickfromwales]

29 minutes ago, RodW said:

 

 

That is an interesting point.  I assumed we would just move the space heating over to the ASHP but this would allow the heating to work with the TS if its hot enough. There would a link to the ASHP controller calling for heat when there is demand in the building and the TS is too cold.  Would there be a volumiser attached to the ASHP that the diverter valves connect to?

Yup. Quite easy to implement. 
A 50L energy buffer in-line with flow might be advantageous, dependant on the minimum loads that the house may call for vs the manufacturers installation guide.