Another new member and ASHP in Somerset

[Chris Bottomley]

Another new member. I've been following the forum for a while with regard to heat pumps and thought it was about time to join in.

 

We have a 1960's large detached 4 bed chalet bungalow of 173m2 floor area in Somerset.  It is insulated as well as possible for a house of that age without deconstructing the house, it has recent double glazing in most windows, cavity wall insulation and 250 to 300mm insulation in roof voids where possible and 75mm PIR in dormer and pitched ceiling spaces. The EPC space heating requirement 16750kWh and 3500kWh for water heating.  Heating and hot water was supplied by a 28kW oil boiler feeding 13 radiators mainly single panel of 1970's vintage and a 12m2 underfloor heating loop in a sun room.

 

The oil boiler was replaced by a 12kW LG ThermaV Monobloc ASHP system in May 2020.  The existing hot water tank was replaced by combined hot water and heating buffer tank.  Eight of the existing radiators were replaced with new unts of double the existing capacity and two further radiators fitted based on a heat loss survey. Controls are the LG remote control unit and a single programmable thermostat.  The underfloor heating loop runs off the main heating loop as a separate zone under control of a second thermostat.  An MMSP monitoring system was fitted but unfortunately the installer didn't get their act together to enable us to get the additional RHI payment.  The monitoring has been useful however in allowing us to work out what the system is doing and when.

 

Our objective when installing the system was to replace the 25 year oil boiler with a system that at a minimum would cost less to run.  We didn't have high expectations of great savings due to the age of the building and consequent insulation levels.  The installation costs, when the RHI payments are taken into account, were probably not much greater than replacing the current oil boiler and 50 year old hot water tank and reworking the hot water system.

My current objective is to optimise the performance of the ASHP system now that it is running in coldish weather for the first time.

[Roundtuit]

Welcome!  There's a load of real-world ashp experience here, so hopefully you can find something of use.

Edited January 22, 2021 by Roundtuit
typo

[Ultima357]

Are you sure that ASHP is large enough to cope? Taking out a 28kw oil boiler and replacing with just 12kw doesn't sound quite right to me. Did they do a heating load requirement for your house? I've got a 12kw Samsung ASHP in 253m2 passive house (new build) but our total heat load is just 7kw.

[PeterW]

3 minutes ago, Ultima357 said:

Taking out a 28kw oil boiler and replacing with just 12kw doesn't sound quite right to me.

most boilers are sized for DHW load not heating load. 28kW is only normally used to give rapid reheat to hot water tanks and the initial surge of a heating system. 

[Ultima357]

 

10 minutes ago, PeterW said:

most boilers are sized for DHW load not heating load. 28kW is only normally used to give rapid reheat to hot water tanks and the initial surge of a heating system. 

Yes I guess so, but 12kw still seems low. His space heating requirements are 4 times higher than mine which OK, can just mean it'll run 4 times longer, but it'll all depend on the actual heat load that they should have calculated given whatever u values they now have. Flowing at 20l/min which seems to be a common need for ashp, 12kw only raises it by 5deg above input temp, so if whole house demand is high, it might struggle to warm up enough. As I have a lot of UFH loops, (nearly 1.5km), when more than half are active it drops the temperature achieved at ashp outlet. 

[Chris Bottomley]

Apologies for the delay in replying, I haven't come back here recently.

 

We had a heat loss survey done and the ASHP was sized by the installers based on that.  The ASHP does run most of the time when the outside air temperature is near zero but it can heat the house well and acheive 20 deg plus internal temperatures if a high outlet temperature used (e.g. 50 deg).   The interesting challenge is setting it up to get adequate heating at lower outlet temperatures to improve its efficiency, we can still get 20 degrees plus if required but it takes longer to get there.  

 

The sizing of the ASHP system seems OK as long as you don't expect to get the same sort of heating response as you would with an oil boiler rated at over twice the ASHP capacity and running a radiator circuit at 80 degrees rather than 40.  The ASHP system is set to a more or less constant internal temperature demand when the heating is on as opposed to the oil system where the heating was off overnight, the oil system could of course raise the temperature back up in a couple of hours.

 

You have to accept that if you have an ASHP the heating system has to be programmed differently and understand that the response time will be different.