Optimising a heat pump

70sbudgie

I have finally got to the point where I can do a cost comparison. 

If I were to pay 24.5p/kWh for all my heating and hot water, the HP would be between £80 and £150 per year more expensive than the gas boiler at ~ 6.5p/kWh for gas. (Flow temp 43°C-47°C).

If I were to pay 13p/kWh (the lowest Cosy rate for my area), I could save £500-£540 a year.

So, all the extra radiator changes to enable the design temp to drop by 4°C, has significantly less impact than the tariff. 

The break even point for the flow temp of 47°C that I've been quoted is ~22p/kWh. 

Reed_Richards

When I got my heat pump in December 2020 the installer made a cost comparison but this was based on electricity costing 14p per kWh.  Now it's only a little over 4 years later and how the situation has changed!

When you compare running costs you really need to make some sort of estimate about how prices will change in future.  If governments are serious about cutting greenhouse gas emissions they will load more charges onto gas to raise its price relative to electricity.  But it doesn't look to me as if any government will have the nerve to do that.

FreeBear

70sbudgie said:

For me, that is what makes getting my head around HP so challenging - there are multiple points where a delta T exists. It can be used to describe how much energy is available to input into the system (1 outside air temperature Vs out flow temp), how much energy is required to be taken out of the system (2 room temp Vs emitter in temp), how much energy a particular emitter can emit (3 in and out flow temperatures) and then how much energy needs to be put into the system (4 flow and return temperatures).

Throw in calculations to determine how big the pipes need to be to carry that heat adds to the headaches. I had thought that using 22mm for the main feed & returns would be big enough, but back of fag packet calculations suggests that I might need 28mm as far as the first three or four radiators.

70sbudgie

Yes. But there are some things that I'm not prepared to change - towel rails fitted to tiled walls, or existing pipework (which appears to all be copper at least the diameter of my finger). So I can at least remove some variables. 

Trying to use Heat Punk to match the quote I was given, I have noticed a few things. Our ceilings are not standard height - they are 10cm lower than the default 2.4m. That was the one change that probably had the biggest impact on heat loss modelling. And accurately measuring the windows also made a big difference. 

Alnat1

70sbudgie said:

Yes. But there are some things that I'm not prepared to change - towel rails fitted to tiled walls 

  

We had bigger towel rails fitted in the bathroom and en suite. Both rooms have tiled walls but a bit of filler soon sorted the holes. A used teabag dabbed on a couple of hours later sorted the colour match (luckily both beige shades) and you really can't tell unless you go looking for it.

We've also bought cheap towel hooks that fit over the bars. IKEA towels have hanging loops, so using these means less of the radiator is covered but they still dry well.

NedS

70sbudgie said:

Yes. But there are some things that I'm not prepared to change - towel rails fitted to tiled walls, or existing pipework (which appears to all be copper at least the diameter of my finger). So I can at least remove some variables. 

Trying to use Heat Punk to match the quote I was given, I have noticed a few things. Our ceilings are not standard height - they are 10cm lower than the default 2.4m. That was the one change that probably had the biggest impact on heat loss modelling. And accurately measuring the windows also made a big difference. 

Air changes per hour also have a massive impact, and that is one of the things you probably know least about (unless you perform a door blower test). They tend to use a value of 1, which is probably on the high side for a well insulated house with modern double glazing, but MCS seem to recommend a value of 1 across the board for all house types when calculating the heat loss. A house with old wooden windows and open chimney / fireplaces will have much higher air changes than a modern house with new uPVC doors and windows where a value of 0.5 is probably more realistic.

I would try to measure the actual heat loss from actual gas usage on a known day, then adjust settings in heat punk to get the figures to roughly match. What heat loss calculations are really useful for (in my opinion) is giving the relative heat loss for different rooms, so you can size radiators appropriately in each room. Even if the overall figure is high (or low), if the rooms are all correctly proportioned then you can run the system wide open without TRVs which is most efficient, rather than having a cold room that never gets up to temp, or an overly warm room that needs throttling with a TRV because the radiator is too big for the room. If the overall heat loss is out, it just affects the flow temps you'll need to run at, but at least the system is in balance.

Reed_Richards

In our bathroom we already had a towel rail but we added a second radiator that only stands 35 cm off the floor.  This and a coat rail above created a little area for drying or airing clothes.  My OH was very pleased with this and it meant we didn't have to change the towel rail.

70sbudgie

Alnat1 said:

70sbudgie said:

Yes. But there are some things that I'm not prepared to change - towel rails fitted to tiled walls 

  

We had bigger towel rails fitted in the bathroom and en suite. Both rooms have tiled walls but a bit of filler soon sorted the holes. A used teabag dabbed on a couple of hours later sorted the colour match (luckily both beige shades) and you really can't tell unless you go looking for it.

We've also bought cheap towel hooks that fit over the bars. IKEA towels have hanging loops, so using these means less of the radiator is covered but they still dry well.

I also use hooks over the towel rails too. And when choosing the towel rails (at various points over the last 15 years) I tried to choose ones that had higher output, so they are either really tall, or have panels rather than bars. 

70sbudgie

NedS said:

70sbudgie said:

Yes. But there are some things that I'm not prepared to change - towel rails fitted to tiled walls, or existing pipework (which appears to all be copper at least the diameter of my finger). So I can at least remove some variables. 

Trying to use Heat Punk to match the quote I was given, I have noticed a few things. Our ceilings are not standard height - they are 10cm lower than the default 2.4m. That was the one change that probably had the biggest impact on heat loss modelling. And accurately measuring the windows also made a big difference. 

Air changes per hour also have a massive impact, and that is one of the things you probably know least about (unless you perform a door blower test). They tend to use a value of 1, which is probably on the high side for a well insulated house with modern double glazing, but MCS seem to recommend a value of 1 across the board for all house types when calculating the heat loss. A house with old wooden windows and open chimney / fireplaces will have much higher air changes than a modern house with new uPVC doors and windows where a value of 0.5 is probably more realistic.

I would try to measure the actual heat loss from actual gas usage on a known day, then adjust settings in heat punk to get the figures to roughly match. What heat loss calculations are really useful for (in my opinion) is giving the relative heat loss for different rooms, so you can size radiators appropriately in each room. Even if the overall figure is high (or low), if the rooms are all correctly proportioned then you can run the system wide open without TRVs which is most efficient, rather than having a cold room that never gets up to temp, or an overly warm room that needs throttling with a TRV because the radiator is too big for the room. If the overall heat loss is out, it just affects the flow temps you'll need to run at, but at least the system is in balance.

I did struggle to get my head around the air changes. I think this is where my Heat Punk model probably deviates most from the installer's. We have a large extension that is less than 10 years old, so was completed to a different set of building regs to the original building. I got the impression from the EPC surveyor that it is the air changes that make the most difference between the two ages of construction (we had cavity walls filled about 15 years ago, so the older building is pretty well insulated)

I've used my Heat Punk model to identify additional radiators to be replaced and have asked the HP designer what impact those changes would have. 

70sbudgie

On trying to find out more information about air changes, I came across this interesting article about a change to the ventilation calculations for an MSC compliant design from this June. 

https://heat-box.uk/en-gb/blog/post/new-mcs-requirement-heat-loss-calculations-must-follow-new-standard-from-june-2025