Improving EPC Rating before fitting a Heat Pump

[Quisling]

Thread revival.

If you buy an old house with solid stone walls, what do you do?  External insulation would ruin all its appeal, and internal insulation reduces the space and means enormous disruption as all woodwork, electrics, etc, have to be moved.

The need for insulation (which would be of equal benefit with any form of heating, in a pure heat loss sense) seems to be because heat pumps aren't good at bringing a house up from cold.

Biomass might be a better option, but they can turn you down for a grant if you don't meet the insulation standards.

Heatpumps (and biomass solutions) will be closely sized to match the building heat loss to avoid inefficient and equipment-life-shortening cycling on/off.  They are also typically designed with a 5C delta between flow/return temperature. Accordingly, they have minimal spare capacity to bring the house up to temperature, especially at times of peak demand in cold weather, and are therefore best operated on a "low and slow" basis.  We have a wood pellet biomass boiler, and it's set to run as required by weather compensation controls throughout the winter, with a night set-back target temperature only about 3C below the day target otherwise it can struggle to get the house warm again the next day when it's v cold.

There are two obvious solutions to this problem for heat pumps
1. Install a heat pump as lead "boiler" but retain a gas boiler as "top-up" - i.e. operate a hybrid system.  This avoids the need to replace radiators if you still need to operate at higher flow temperatures.  This is probably not eligible for BUS grant though (I've not checked).
2. Install a two stage heat pump system. This allows overall system over-sizing without introducing rapid cycling on/off, and delivers some system redundancy.  However, it adds significant cost and some control complexity but is probably worth it for large properties.  Using two heat pumps with different refrigerants may maximise efficiency - allowing a low temperature heat pump to operate most of the time and deliver excellent seasonal COP, while allowing a higher temp heat pump to operate efficiency when required for top up heat (or to deliver domestic hot water at 60C)

The need for insulation improvements is only part of the problem here, and can be significantly overstated.  Total heat loss from a property is a combination of building fabric heat losses, and ventilation heat loss - which can be significant in older properties.  Some ventilation is required to introduce fresh air (debatable how "fresh") and prevent build-up of moisture.  However, measuring ventilation heat-loss is beyond the wit of typical EPC assessors so they use default values and focus on what they can see which is the building fabric.  You can have a proper air-tightness test done (e.g. https://airtightnesstesting.co.uk/pricing/) and this is now a requirement for new properties under the Part L Building Regs.  I think if you had evidence of low air change rate that would help mitigate requirements for disproportionately costly upgrades to insulation in hard-to-treat locations. 

With respect to biomass possibly being a better option my advise is "don't".  They will still be sized close to the total building heat demand so will still take a long while to heat the property up from cold in the winter.  Moreover, the cost of servicing is heinous - mine is now 10 years old and the standard service without major parts or repairs is at least £425 a year, compared to maybe £100 for an oil boiler.
10 years ago, heat pumps aren't what they are now.  If I was replacing the system again tomorrow I'd go down the heat pump route.

[HTFB]

Heatpumps (and biomass solutions) will be closely sized to match the building heat loss to avoid inefficient and equipment-life-shortening cycling on/off.  They are also typically designed with a 5C delta between flow/return temperature. Accordingly, they have minimal spare capacity to bring the house up to temperature, especially at times of peak demand in cold weather, and are therefore best operated on a "low and slow" basis.  We have a wood pellet biomass boiler, and it's set to run as required by weather compensation controls throughout the winter, with a night set-back target temperature only about 3C below the day target otherwise it can struggle to get the house warm again the next day when it's v cold.

There are two obvious solutions to this problem for heat pumps
1. Install a heat pump as lead "boiler" but retain a gas boiler as "top-up" - i.e. operate a hybrid system.  This avoids the need to replace radiators if you still need to operate at higher flow temperatures.  This is probably not eligible for BUS grant though (I've not checked).

It isn't.

2. Install a two stage heat pump system. This allows overall system over-sizing without introducing rapid cycling on/off, and delivers some system redundancy.  However, it adds significant cost and some control complexity but is probably worth it for large properties.  Using two heat pumps with different refrigerants may maximise efficiency - allowing a low temperature heat pump to operate most of the time and deliver excellent seasonal COP, while allowing a higher temp heat pump to operate efficiency when required for top up heat (or to deliver domestic hot water at 60C)

I'm not sure how many of these are on the market. Anyway it is debatable whether your HP should regularly be heating water to 60C, as it's terrible for your COP. I think it's more usual to set to 50-55C with only a weekly anti-legionella cycle. Isn't it? Even if the HSE don't like it.

[rogerzilla]

My lifestyle doesn't suit "low and slow".  I want no heating at night, because I can't sleep unless it's cool, I don't really need heat first thing in the morning, and I certainly don't want to be heating the house when I'm out.

This is what really scares me about heat pumps: having to waste heat all the time (or even open windows) because that's what suits the system.

[HTFB]

My lifestyle doesn't suit "low and slow".  I want no heating at night, because I can't sleep unless it's cool, I don't really need heat first thing in the morning, and I certainly don't want to be heating the house when I'm out.

This is what really scares me about heat pumps: having to waste heat all the time (or even open windows) because that's what suits the system.

That is very much a Britain Problem. Everywhere else in the world people can sleep with the windows shut. Which is not to say it's not a real issue for people with long-entrenched sleeping habits.

But heating the house cheaply while you're out is better than heating it expensively when you're back, especially if it means being warm from the moment you're in rather than having to wait for it to heat up again. This one is just a prejudice to reconsider. Think of it as a storage heater, people understand those.

[rogerzilla]

Heating it for 9 hours while you're at work 5 days a week?  Thst's insane compared to getting it up to temperature shortly before you get home (you don't come home to a cold house, as there are things called programmers, or Hive if your movements are less predictable).

My guess is that the energy comparisons are made with a gas boiler running at constant thermostat temp, which few people do (or can afford to do).

You can't change the laws of thermodynamics: a house's heat loss is in proportion to the inside/outside temperature difference.

[Kim]

Cool bedrooms should, in principle, be achievable by not heating the bedrooms.  But the only thing less insulated than the exterior of a BRITISH house, is the interior.

Sleeping on the ground floor and living on the first floor would help, but that's like eating your desert first.

[Diver300]

Heating it for 9 hours while you're at work 5 days a week?  Thst's insane compared to getting it up to temperature shortly before you get home (you don't come home to a cold house, as there are things called programmers, or Hive if your movements are less predictable).

My guess is that the energy comparisons are made with a gas boiler running at constant thermostat temp, which few people do (or can afford to do).

You can't change the laws of thermodynamics: a house's heat loss is in proportion to the inside/outside temperature difference.

If the room temperature drops significantly during a few hours while you are at work, the insulation is terrible.

Unfortunately, that would be the norm in a British house.

[HTFB]

Heating it for 9 hours while you're at work 5 days a week?  Thst's insane compared to getting it up to temperature shortly before you get home (you don't come home to a cold house, as there are things called programmers, or Hive if your movements are less predictable).

The rate of heat loss from your home is, as you so rightly say, proportional to the difference between inside and outside temperatures. Say on a cold day, at 0C outside, the temperature drops from 20 to 15 while you are out. Ignoring a bit of convexity that's an average temperature difference of 17.5 degrees. Keeping the place warm while you're out is an average of 20 degrees. So you'll be losing 20/17.5 - 1 = 14% more heat by leaving the heating on. If delivering the heat over 9 hours is more than 14% more efficient than delivering it over your 1 or 2 warm-start hours -- as it will be, with a right-sized heat pump -- you are winning.

If it drops to much below 15 degrees you should probably be spending a bit on insulation or draughtproofing.

[rogerzilla]

My normal practice on winter work days was to only heat in the evening, say from 1600 to 2100.  Cycling to work meant avoiding any need to shower in a cold bathroom.  Over 19 hours, I think the indoor temp dropped to about 13 deg C on really cold days.  This may be a marginal use case.  To be honest, the gas CH struggles to get up to temp from 13 deg, as radiators are never big enough for boiler output!

[quixoticgeek]

My lifestyle doesn't suit "low and slow".  I want no heating at night, because I can't sleep unless it's cool, I don't really need heat first thing in the morning, and I certainly don't want to be heating the house when I'm out.

This is what really scares me about heat pumps: having to waste heat all the time (or even open windows) because that's what suits the system.

It's better to keep things at a constant temp than keep varying the temp.

Sleep cold? Try a thinner duvet...

Having the building drop down to below 15° feels like a recipe for damp.

In an ideal world we'd all love in ultra efficient homes with great insulation so the heat pump barely has to do anything to keep the home at 20°, then when you get home from work you can have it give a little boost to a comfortable temperature.

I discovered recently that many Germans seem to believe that sleeping with the window open is necessary in order to not die of respiratory something or other. So you find the radiator working over drive and a window open. I know that in an average bedroom if I close the door while I sleep I'll wake with a headache due to Co2 buildup. But I solve that by sleeping with the door open rather than the window. I do think people should be more aware of the existence of heat recovery ventilation systems tho.

Gas fired central heating is not sustainable and its days are numbered. Whether you replace that with a heat pump, or district heating, or something else, time will tell. But soon (hopefully), burning Dino farts for heat will end.

J

[orraloon]

In Ecosse, a hybrid combo of ASHP with top up from current (or indeed replacement) gas boiler for when it's Cold does comply with terms of renewables funding scheme.  Well this month anyway.  Though no funding for a gas boiler of course.

[HTFB]

I do think people should be more aware of the existence of heat recovery ventilation systems tho.

It is possible to retrofit MVHR, as I know because we are doing it. But I wouldn't wish on anyone a house in the sort of condition where it makes sense to try, unless you like exposed pipes. More or less the only inside bits we've kept are the joists*, and even they get in the way of the ductwork.

*also the stairs and two or three of the internal walls that didn't fall down.

[HTFB]

o be honest, the gas CH struggles to get up to temp from 13 deg, as radiators are never big enough for boiler output!

If it's a condensing boiler, btw, this will also be costing you efficiency even on gas. You want the return temperature to be low enough that it can condense the steam out of the boiler exhaust, which means below 54 degrees and as much below that as possible.

[rogerzilla]

I think very few condensing boilers are run at full condensing temperature, because of the radiator size problem.  The Potterton boiler I once had ran at either 63 or 82 deg C.  The quoted efficiency was presumably at 63 deg C.  It was factory set to 82 deg C for effective heating.  To change it to 63, you had to open it up and move a jumper on the main circuit board.  I tried it for a week snd the house never got warm.

UK houses are small and have small radiators so there is still somewhere to put the furniture.  The assumptions made in radiator sizing are far too optimistic and assume (like a heat pump) that you run the thing 24/7 so there is no additional "heat things up"* demand, merely covering steady losses.

*air heats up very fast; walls and furniture don't

[aidan.f]

We usually run our boiler at 40, when it gets very cold I have to tweak it up to 50. Guess we are just about good to go with a heat pump.