Alternatives To An ASHP

[Matt60]

I keep trying to find an alternative to an ASHP as I would like a system that operates as quickly and to the temperatures of a traditional system. I have a few old injuries that sometimes need 23 degrees in order to feel comfortable and I'd like to be able to reach that from a sensible starting temperature with a couple of hours. My last oil boiler could do that but I'm building a new house and don't really want to put that dirty old solution in to an otherwise modern house unless I have to.

 

Every time I think I've cracked it, I get a load of information that makes me doubt it will work.

 

The most recent idea was to have PV panels (I can get 4 kw on the roof) and use them to primarily power a big Sunamp with any surplus going to a separate one to heat upstairs radiators. Additionally, I thought I might use 2 Willis heaters to power the underfloor heating. I made and online enquiry with my nearest Sunamp supplier and then received a call from someone who followed up his verbal tripe with the following by email which I have copy/pasted. I have put the parts in red that amused me the most;

 

Good to chat with you, in summary:

 

1. Solar panels will only give you back around £200-300 per year, based on roof direction, and based on a 4kw system (max on a  residential roof), adding a battery will help. I think you are probably looking at a 25-30 year payback, before you see any financial benefit.

 

2. Sunamp - instant hot water system is around £4000-6000 and can be ran off solar, but even without solar it is still an efficient hot water system to run. I am not sure about using the Sun amp to run wet radiators, personally the less water in the house the better, because water leaks are always a lot of trouble.

 

3. Underfloor heating - wet is not too expensive to run but when it goes wrong (and it does) it’s a nightmare as can flood the house. Electric underfloor heating is great but very expensive to run, and when it breaks down, it’s also difficult to trace the issue and involves ripping up floors

 

4. German style electric storage heaters / radiators - they are modern, slimline, energy efficient and totally controllable. They come on and off as you require via wifi and are controlled via an app. There is a lot of choice in regard to sizes and colours, and they come with a 30 year guarantee. Prices range from £800-1500 per heater typically.

 

He sells items 2 & 4 only - you'd never guess would you...

 

From what I have read, a 4kw system should provide 3400kwh per annum and I make that nearer £600 per year if you can use use all the power with batteries or a Sunamp. Is this right? Additionally, I'm not looking for or asked about "pay back". I'm happy to spend a few quid to keep my bills under control, not least in preparation for when I retire.

 

Does anyone know if my above suggested scheme (in orange above) is a reasonable idea? The reason for me separating the two heating circuits is that the radiators would presumably need to run at a higher temperature that the underfloor circuit. The house I'm building will have 150mm fully filled cavities, 150mm Celotex over the block and beam floor, triple glazing and a zinc roof. I'm aiming for a high level of air tightness and intend on using a MVHR system, possibly with some sort of cooling function. I won't have the SAP report for another couple of weeks.

 

My main drainage runs are in and services are being brought in on the 15th of March. I know or at least think I know what I'm doing for the most part, but bar far and away the biggest headache is choosing a heating system. Any advice re my suggested system or an alternative would be appreciated.

 

 

[SteamyTea]

Is this for an existing house or a new build one?

 

PV can realistically only supplement heating, not replace it, in a domestic setting.

 

There is nothing to stop you having an ASHP, just oversize it a bit more than normal, and then supplement with some panel or fan heaters.

Fan heaters will warm a room up pretty quickly.

 

It is worth have PV anyway and, because of the magic of electricity, the electrons will go to the nearest load first and reduce imports.

You can have more than 4 kW_p if your DNO agrees.

Edited February 25, 2021 by SteamyTea

[Matt60]

8 minutes ago, SteamyTea said:

Is this for an existing house or a new build one?

 

PV can realistically only supplement heating, not replace it, in a domestic setting.

 

There is nothing to stop you having an ASHP, just oversize it a bit more than normal, and then supplement with some panel or fan heaters.

Fan heaters will warm a room up pretty quickly.

 

It is worth have PV anyway and, because of the magic of electricity, the electrons will go to the nearest load first and reduce imports.

You can have more than 4 kW_p if your DNO agrees.

Hello, Thanks for your reply.

 

It's a new build, foundations should go in at the end of next month.

 

I have looked at ASHP and thought that if I had one I would just make it bigger than I need to be safe, but then I read that they short cycle if you do that. I like the reliability, speed of heat, and lack of servicing re the Sunamps systems. Is my suggested system in orange no good at all then?

 

I had assumed that electric panel or fan heaters would be quite expensive. I would prefer it all to touch of a button ideally, or better still from my phone if possible.

 

The PV has been in the mix from the outset, my south facing roof space is limited to 14 panels though which I make 4kw with high performing panels. 

[ProDave]

Serious suggestion.

 

Air source heat pump to under floor heating.  This will keep your house warm all the time, in a well insulated modern house that retains it's heat well.

 

Then a well placed (i.e central) wood burning stove.

 

This is what we have.  Constant 20 degrees form the ASHP, but if we want to indulge, light the stove, open all the doors so the heat circulates throughout the house, and it's quite possible to quickly get the house up to 23 degrees or more.

 

[SteamyTea]

24 minutes ago, Matt60 said:

It's a new build, foundations should go in at the end of next month.

How much insulation have you designed into the foundation.  This is quite important, but more so if you do go the UFH route.  You may have to halt the build until you have redesigned the floor.

24 minutes ago, Matt60 said:

I have looked at ASHP and thought that if I had one I would just make it bigger than I need to be safe, but then I read that they short cycle if you do that.

Depends on the ASHP, some can modulate down to quite low levels, and a buffer tank can be oversized to cope with the short cycling issue.

24 minutes ago, Matt60 said:

had assumed that electric panel or fan heaters would be quite expensive

Very cheap to fit, and if the UFH/ASHP combo has already heated the place up to 21°C, resistance heating can quickly raise it 2 or 3°C more.  You would only be heating the air, which has low density, around 1.25 kg.m^-3. and a specific heat capacity 1 kJ.kg^-1.K^-1.

24 minutes ago, Matt60 said:

I would prefer it all to touch of a button ideally, or better still from my phone if possible.

There are many different controllers that can now work with phones.  This part is really separate from the underlying heating technology.

 

The Sunamp was never really designed to be part of a central heating system.  It works by changing phase at a constant temperature, and can be tricky to part charge, especially from an intermittent source like PV.  A larger than normal buffer tank with some electrical immersion heaters in it can work pretty well.

Edited February 25, 2021 by SteamyTea

[Matt60]

Thanks, I see.

 

The house I'm building will have 150mm fully filled cavities, 150mm Celotex over the block and beam floor, triple glazing and a zinc roof. I'm aiming for a high level of air tightness and intend on using a MVHR system, possibly with some sort of cooling function. I won't have the SAP report for another couple of weeks.

 

Is my suggested system in orange no good at all then?

[Matt60]

4 minutes ago, ProDave said:

Serious suggestion.

 

Air source heat pump to under floor heating.  This will keep your house warm all the time, in a well insulated modern house that retains it's heat well.

 

Then a well placed (i.e central) wood burning stove.

 

This is what we have.  Constant 20 degrees form the ASHP, but if we want to indulge, light the stove, open all the doors so the heat circulates throughout the house, and it's quite possible to quickly get the house up to 23 degrees or more.

 

Would you use the same pump for the radiators upstairs too? I can see the logic in what you're saying, thanks.

 

I am interested to know if my suggested system in orange warrants any further investigation?

 

I do intend having a bottle fed open flame fire in the living room, but this is separate to the main house, accessed via a short half glazed link and with a vaulted ceiling with no rooms above it. I want a bottle fed fire as they are very controllable and I don't have to clean it every day and gather fuel etc.

[SteamyTea]

1 hour ago, Matt60 said:

The most recent idea was to have PV panels (I can get 4 kw on the roof) and use them to primarily power a big Sunamp with any surplus going to a separate one to heat upstairs radiators. Additionally, I thought I might use 2 Willis heaters to power the underfloor heating

By using just resistance heating from your PV, even if it was working at maximum output for 2 hours a day, would only give you 8 kWh/day.

If you put that same energy into a heat pump, you would get 16 to 24 kWh a day.

It all works on the laws of averages, some days you will not get much generation, and other days you will get too much, but with a heat pump you can maximize what you do generate.

 

150mm of insulation under the floor should be OK, the more the merrier as you don't want to be heating the ground or any air void under the house.

 

Your MVHR will not help a great deal with cooling.  MVHR is not a heating system, it is just a low loss ventilation system.

 

When you get your SAP back, it should be easier to design a system that is compatible with your house.

 

Edited February 25, 2021 by SteamyTea

[pdf27]

Disjointed thoughts since it's late and I spent 4 hours on the phone today:

• I'm really not convinced a Sunamp is any better than an unvented hot water cylinder for 90% of people - it's a lot more money for a small space saving. Unless you're building on a very compact site you're going to be much better off with a cylinder. For the capacity they're quoting 4-6x the price of a comparable cylinder.
• I'd suggest a fan heater or something would be a far cheaper way to be safe - you're very unlikely to need the safety if you do your sums right and it'll cost you a lot of money up front to build in that safety margin with a heat pump. If you're fitting MVHR you can get post heaters which go in them for a few hundred pounds, capable of providing quite a bit of extra heat (the Passivhaus standard is essentially written around one of these heating the whole house).
• I can't remember the last time I saw a wet heating system leak - I've got 1970s pipework at the moment which has all sorts of problems but leaking isn't one of them. Sounds like an excuse to sell you a dry heating system.
• I would question the value of being able to control the heating system from your phone - it sounds like a lot more hassle than a heating system that just sits there and does it's thing without your intervention. With a well insulated house, there really isn't much benefit to turning the thermostat down during the day (particularly if PV is most available then), so you may be as comfortable and have less to do with a very simple thermostat.
• The bit about "German style electric storage heaters" is pure snake oil.
• I wouldn't worry about two circuits - oversize the radiators upstairs as much as you can (good for heat pump efficiency), and you'll presumably have a mixing valve for the underfloor heating anyway to bring the flow temperature down from that in the upstairs circuit to what you need. Remember that heat will rise within the house so you shouldn't need as much heat from upstairs radiators as you would from ground floor ones.
• Several people on here use Willis heaters for the underfloor heating, with Economy 7. The only reservation I have is about your insulation - 150mm of mineral wool isn't all that much (~0.23 U-value) and using direct electric heating might be expensive, those who do it are at or close to Passivhaus. Check the heat loss calculation before you decide.

[Matt60]

1 minute ago, SteamyTea said:

By using just resistance heating from your PV, even if it was working at maximum output for 2 hours a day, would only give you 8 kWh/day.

If you put that same energy into a heat pump, you would get 16 to 24 kWh a day.

Is a Sunamp considered resistance heating or would that give similar numbers to to the ASHP?

3 minutes ago, SteamyTea said:

150mm of insulation under the floor should be OK, the more the merrier as you don't want to be heating the ground or any air void under the house.

I have just got time to change this and I did discuss it but I was told that it needs to be in multiples of 75mm to match the brick courses. 225mm would be difficult and 300mm a bit excessive and expensive?

[Matt60]

18 minutes ago, pdf27 said:

Disjointed thoughts since it's late and I spent 4 hours on the phone today:

• I'm really not convinced a Sunamp is any better than an unvented hot water cylinder for 90% of people - it's a lot more money for a small space saving. Unless you're building on a very compact site you're going to be much better off with a cylinder. For the capacity they're quoting 4-6x the price of a comparable cylinder.

That's interesting and I have plenty of space for a cylinder, I was more thinking of the Sunamp as way to use all the PV energy.

 

18 minutes ago, pdf27 said:

I'd suggest a fan heater or something would be a far cheaper way to be safe - you're very unlikely to need the safety if you do your sums right and it'll cost you a lot of money up front to build in that safety margin with a heat pump. If you're fitting MVHR you can get post heaters which go in them for a few hundred pounds, capable of providing quite a bit of extra heat (the Passivhaus standard is essentially written around one of these heating the whole house).

I'll look in to that, I'm not keen on the fan heater though tbh, I'd like it as a nice neat centrally controlled system if possible.

 

18 minutes ago, pdf27 said:

The only reservation I have is about your insulation - 150mm of mineral wool isn't all that much (~0.23 U-value) and using direct electric heating might be expensive, those who do it are at or close to Passivhaus. Check the heat loss calculation before you decide.

The insulation is Dritherm 32, I suppose that is mineral wool. I could just about change it, I originally asked for a 200mm cavity but the architect, bricklayer and builders merchant all said it would cause issues with Catnics that could span a 200mm cavity. When I suggested separate steels or concrete lintels I was told it would be costly to have the steels as they would all have to be individually calculated and the steel supply could delay things. The builders merchant said there is no off the peg Catnic type steels that would suit.

Edited February 25, 2021 by Matt60

[LA3222]

15 minutes ago, Matt60 said:

That's interesting and I have plenty of space for a cylinder, I was more thinking of the Sunamp as way to use all the PV energy.

You seem to be trying to find reasons to justify using a SunAmp setup. A better way to approach this may be to step back from the SunAmp and ask the question as to what may be the best way to skin this cat.

 

You can use up all the PV for a far better return via an ASHP. The ASHP can heat your DHW and your CH load. If you have a PV diverter then the DHW can be heated to 60 odd degrees. Use a buffer tank for your UFH and get this up to 40 odd degrees which should heat your house up to 23 degrees easy enough if that's the temp you want.

 

I'm not saying this is the optimal solution for you, just another approach to consider.

Edited February 25, 2021 by LA3222

[SteamyTea]

8 hours ago, Matt60 said:

Is a Sunamp considered resistance heating or would that give similar numbers to to the ASHP?

 

It is resistance heating.

8 hours ago, Matt60 said:

300mm a bit excessive and expensive?

You only pay that once, at todays prices.  Heating you pay forever, at future prices.

7 hours ago, LA3222 said:

A better way to approach this may be to step back from the SunAmp and ask the question as to what may be the best way to skin this cat.

Carefully, and not in front of children.

[Moonshine]

8 hours ago, pdf27 said:

The only reservation I have is about your insulation - 150mm of mineral wool isn't all that much (~0.23 U-value)

 

With lightweight blocks and dritherm this comes to about 0.20.

It's what I am looking to build. I think it drops by 0.01 if you use aerated blocks internally.

[Ian]

8 hours ago, Matt60 said:

 

I have just got time to change this and I did discuss it but I was told that it needs to be in multiples of 75mm to match the brick courses. 225mm would be difficult and 300mm a bit excessive and expensive?

300mm of EPS insulation would be the cheapest way to improve things and would result in a U value of about 0.1.

[Moonshine]

12 minutes ago, Ian said:

300mm of EPS insulation would be the cheapest way to improve things and would result in a U value of about 0.1.

 

Maybe in materials, but you will have to consider the extra excavation and muck away costs.

[ProDave]

9 hours ago, Matt60 said:

Would you use the same pump for the radiators upstairs too? I can see the logic in what you're saying, thanks.

 

I am interested to know if my suggested system in orange warrants any further investigation?

 

I do intend having a bottle fed open flame fire in the living room, but this is separate to the main house, accessed via a short half glazed link and with a vaulted ceiling with no rooms above it. I want a bottle fed fire as they are very controllable and I don't have to clean it every day and gather fuel etc.

Seriously, think again about radiators upstairs.  That will force an ASHP to run at a hotter flow temperature than it does with UFH and it will be less efficient and more likely in cold weather to defrost.

 

Instead fit UFH upstairs, either using the aluminium spreader plate system, or if you design the joists for the extra dead load, using a pug mix screed system.

 

A lot of us, in very well insulated houses find you don't actually need any heating upstairs.  I provisioned for small electric panel heaters in the bedrooms and have not fitted them, the electric points remain there, unused.

[Gone West]

Strangely I agree with some of the comments from the Sunamp supplier. I wouldn't consider PV as part of my space heating solution. I also have a dry house and I wouldn't choose a wet space heating system in a modern well insulated house. I would separate DHW and space heating systems and use a small dedicated ASHP for DHW and an A2A system for space heating and cooling. There is a member of the GBF who heats their house with A2A and is happy with it.

[jack]

I'd argue that you should essentially ignore PV as part of your heating assessment, as insolation is at lowest exactly when you need the most heating in the middle of winter.

As an example, I have 8.5 kW of PV. It managed 144 kWh in December, and 135 kWh in January. In January, there were many days when I got less than 5 kWh. For resistance based heating, that amounts to covering the energy required to operate a small fan heater for about 2 hours.

 

With a 4 kW system, you're looking at less than half that output - so a small fan heater for an hour.

 

Sure, during sunny days it makes more, and if your insulation is current building regs level, you'll need heating during the shoulder months, and PV will help. I just wouldn't be making PV a significant focus of my heating plans.

[JamesP]

2 hours ago, jack said:

I'd argue that you should essentially ignore PV as part of your heating assessment, as insolation is at lowest exactly when you need the most heating in the middle of winter.

As an example, I have 8.5 kW of PV. It managed 144 kWh in December, and 135 kWh in January. In January, there were many days when I got less than 5 kWh. For resistance based heating, that amounts to covering the energy required to operate a small fan heater for about 2 hours.

 

With a 4 kW system, you're looking at less than half that output - so a small fan heater for an hour.

 

Sure, during sunny days it makes more, and if your insulation is current building regs level, you'll need heating during the shoulder months, and PV will help. I just wouldn't be making PV a significant focus of my heating plans.

We have a 3.74kW PV array and it produced 95 kWh in December and 109kWh in January. What is your average over the year?

[jack]

21 minutes ago, JamesP said:

We have a 3.74kW PV array and it produced 95 kWh in December and 109kWh in January. What is your average over the year?

 

We average around 7 MWh per annum, which is somewhat less you might expect for the same array mounted to a south facing roof at a higher angle, but still pretty good given the angles some of the panels are at.

 

Our panels are on a flat roof and we wanted them out of sight. The panels are therefore all mounted at relatively shallow angles - mostly south-facing, but there's a line of panels with alternating east-west orientation that's actually tilted slightly to the north (as well as being angled east and west). These typically generate about 80% of the energy of the south-facing panels.

 

Interesting that your array produced more in January than December - ours was the other way around this year. We had a particularly poor January this year at 135 kWh - the previous three Januaries were 164, 184 and 161 kWh.   

 

Edited to add: so yes, my previous comment was somewhat pessimistic about how much energy you might expect from the average 4 kW installation, but the general principle of there not being very much energy available in winter compared to heating needs still applies.

[Fenton H]

Have you thought of a backup electrical heater downstream of the ASHP?  Many models (e.g. air-to-water split systems) include them as standard in the indoor hydro unit, typically 3, 6 or 9kW.  Or you can buy as a separate item. The idea is you don't need to oversize the ASHP too much to allow for short bursts of extra heat to get up to temperature, or for the few coldest days at the bottom end of your design envelope.

 

 

[SteamyTea]

25 minutes ago, Fenton H said:

Have you thought of a backup electrical heater downstream of the ASHP?  Many models (e.g. air-to-water split systems) include them as standard in the indoor hydro unit, typically 3, 6 or 9kW. 

Trouble with that is that you may overheat the slab unnecessarily just to get an extra couple of degrees air temperature.

If you take a 4m by 6m room, with the UFH pipes 100mm down, sitting on top of 100mm of another concrete, that slab will take  ~5.5 kWh just to raise the floor up by 2°C.

That is just to get an extra 0.042 kWh into the air.

 

(Divide by the system power to get the time needed to heat up)

Edited February 26, 2021 by SteamyTea

[JamesP]

4 hours ago, jack said:

 but the general principle of there not being very much energy available in winter compared to heating needs still applies.

I do agree with the above. Was just interested and you highlighted the position and location which does have a minimal impact on performance.

Ours is southeast facing, about 35-40 degrees and has no shading.

[pdf27]

19 hours ago, Matt60 said:

That's interesting and I have plenty of space for a cylinder, I was more thinking of the Sunamp as way to use all the PV energy.

 

I'll look in to that, I'm not keen on the fan heater though tbh, I'd like it as a nice neat centrally controlled system if possible.

 

The insulation is Dritherm 32, I suppose that is mineral wool. I could just about change it, I originally asked for a 200mm cavity but the architect, bricklayer and builders merchant all said it would cause issues with Catnics that could span a 200mm cavity. When I suggested separate steels or concrete lintels I was told it would be costly to have the steels as they would all have to be individually calculated and the steel supply could delay things. The builders merchant said there is no off the peg Catnic type steels that would suit.

• A Sunamp and a hot water cylinder with an immersion heater are essentially the same - the difference is that Sunamp use a specialist chemical mix instead of water, which allows them to pack more heat energy into a given volume. That costs more, and in addition of course they're the only supplier for this sort of device so will be pricing at a premium. You can get a box for a few hundred (e.g. the Eddi) which plugs into the immersion heater in a normal cylinder and uses PV in the same way as a Sunamp. The whole shebang shouldn't cost more than half the cost of an equivalent Sunamp.
• If you're going for MVHR, look at fitting a post heater - it's effectively a hidden fan heater, basically all of the supply air from the MVHR flows over it and gets warmed up in the process. Covers the whole house at 25% of the price of the heaters he's trying to sell you, but there is a limit to how much heat it can provide. Unless you're insulating much better than currently planned, it's only going to be good as supplemental heat rather than your only heating source.
• @joe90 can answer this one - he used 200mm Dritherm 32 in a brick and block build, so can tell you exactly how to solve that one cost-effectively.