ASHP design and BUS eligibility

greenbee

We're starting the process of looking at the options for getting rid of the stinky oil boiler and stupidly small oil tank, and talking to ASHP installers.

As we're planning on getting rid of the boiler, we've recently been running it as low as possible, and working on getting it delivering continuous low(ish) heat rather than the whole overshoot/cool down cycle that you generally get with the 70C temperature that oil boilers run most efficiently at. We've managed to get the house pretty stable temperature-wise, at temperatures that are comfortable for us (with the thermostatic valves set on 2-3) and water still sufficiently hot for showers and baths. Downstairs is set at 19-20C and upstairs at 17-18C (set back to 16C and night, but rarely dropping that low).

The designers we have spoken to all seem to work on the basis of room temps of 21C for living rooms, 20C for bedrooms, and 22C for bathrooms - which would require replacing a few radiators. We're not keen on this, partly because we have no need for such high temperatures (we would find them uncomfortable), and partly because of the upheaval and disruption involved.

So my question is - does the BUS scheme mandate specific room temperatures? Or can we specify these ourselves?

We know that the current radiators are perfectly capable for delivering the temperatures we need, and could upgrade them at a later date if we found we needed bigger ones as we age.

tacpot12

The BUS scheme mandates that the system is designed according to the MCS Standards. This mandates certain design parameters to ensure that if you sell the house on, the next owner won't find that they can't heat it to the temperatures they want. The scheme does mandate the design temperature for the inside rooms, see here: Design Conditions - MCS Heat Load Calculator

greenbee

Thanks. Looks like it might have to wait then - we may be better off just moving the boiler flue and buying a larger oil tank. While we'd prefer to move to air source heating, I don't want unnecessary upheaval for something that won't benefit us.

I'll see what comes back, and whether there is any scope to adjust it - and certainly need to make sure that whatever we put in runs efficiently at the temperatures we need.

NedS

We went through a similar process, had an oil boiler which we were able to turn down to 50C (as low as it would go), and found it was still able to keep the house warm in winter thus convincing us that low temperature heating was both practical and possible.

If you go ahead and design for the warmer MCS room temperatures, all that will mean is that you'd require a higher flow temp, say 50C for example. But you'll likely find two things - firstly the MCS heat loss calcs massively overestimate your heat loss, and secondly you will naturally be able to run at lower flow temps because you don't want your house heated to the designated MCS temperatures. Hence you might find that although the system was designed for a 50C flow temperature, it is perfectly happy running at 40C because of these two factors. So don't let it be an issue for you.

Our system was designed with a 50C flow temp at -2C ambient outdoor temp. For the last two winters, we have never run higher than 35C, and we mostly run at 32C flow temperatures all winter to keep the house at 20C. The reason we are able to run so low (32C), is that we changed all of our radiators, fitting the largest radiator each room could reasonably accommodate. If we had stuck with our original smaller radiators, the flow temperatures would have needed to be higher to achieve the same heat output.

Ultimately, running at lower flow temperatures just makes the system even more efficient - the lower the better.

greenbee

Thanks - we’re currently running the boiler at 45C and the room they have identified as needing bigger rads has already been upgraded from one 1400 x 600 double panel single convector to one 1600 x 600 T2 plus two 900 x 600 T2 which at 45C are making the room ridiculously hot unless we open the doors (TRVs on 2)… and it’s a room we don’t want to have to clear again, or decorate (radiators will need painting as a minimum. Given the rest of the ground floor is about to be gutted, it’s going to be the only available living space for some time - so this is really just one step too far in terms of disruption.

I’ll see what the detailed design says - he’s clearly managed to make it work without changing radiators upstairs (where I’d expected it to be problematic). I can only assume that the vent for the woodburner in the sitting room is affecting the calculation. I have a friend who did heatgeek training in preparation for their heat pump project, so will see whether he has any suggestions once I have something to look at.

If it can’t be done, we’ll keep the boiler a bit longer (it’ll need a thorough service!) and make sure that the renovation of the rest of the ground floor includes UFH/rads that will be heat pump ready (including undoing the three zones in the ground floor and making them a single zone) as well as leaving the space needed for the plant room. And revisit the idea of a heat pump when we can cope with clearing the sitting room and dealing with the mess.

tfhnota

Octopus have a heat pump that can run at higher temperatures which might be better suited to the house, but it will run at lower temperatures as well to suit your heating requirements, initial online quote.

greenbee

I’m not going with Octopus for this - I’m being very deliberate about the people I approach based on recommendations from local customers. I need someone who will work with my builders and electricians, and can cope with the complexity of the existing system and the size of property, and will maintain the system (and take on the rest of the plumbing in the house) in future.

But yes, I will talk to him about what flow temp we need to design for in order to meet the criteria based on not changing radiators in the ‘finished’ bit of the house. We’ve already discussed the possibility of a dual unit cascade system to deal with the low heat demand spring/autumn months as the larger units may not modulate down far enough.

QrizB

While we'd prefer to move to air source heating, I don't want unnecessary upheaval for something that won't benefit us.

Larger radiators will allow lower flow temperatures, higher CoP and lower energy bills. The latter should be a benefit to you?

One regular heat pump forum contributor @matt_drummer achieves a CoP of 5 or so, in large part due to his larger radiators.

greenbee

I realise the energy bills might be lower, but I'm pretty sure that we'd get temps we're comfortable with using the existing radiators. And the disruption and upheaval of replacing those will be significant. Not to mention that the cost of making good may well wipe out any potential saving.

Netexporter

https://forums.moneysavingexpert.com/discussion/comment/81882847#Comment_81882847

Have you got a quote from Heat Geeks? (the installer will be local to your area) Their Zero Disrupt methodology makes the minimum of changes to meet a guaranteed SCOP (you can choose standard or enhanced efficiency).

greenbee

https://forums.moneysavingexpert.com/discussion/comment/81882884#Comment_81882884

I think that will be my next step once I've seen the report and raised any queries. Thank you!