Is an electric combi boiler acceptable now for a new build?

[ProDave]

Our plasterer is planning a new build, out in the countryside with no mains gas.

 

I have tried to show him the benefits of an ASHP, he has looked at mine and I have told him the costs and how it works.  But for some reason he is still no convinced.

 

Instead he has got it into his mind he will heat the house and hot water with an "electric combi boiler"

 

So the question, is that likely to pass SAP without any renewable element? 

[PeterW]

Nope... and unless he’s on 3 phase he won’t get an electric combi as he will need 25Kw to water and thats his full load on a single 100A supply. 

[ProDave]

I know where he is building, and I am pretty sure like us there is only a single phase supply available.  We are about 1/2 mile from the nearest 3 phase, the cost of upgrading the 11KV overhead over that distance just so you can get 3 phase would be eye watering

 

I did suggest if he is intent on resistance heating to consider an electric storage boiler and E10 (so it does not have to store heat for so long)  But that does not solve the SAP issue.

 

Perhaps when his architect says it won't pass SAP, he will reconsider an ASHP.

[A_L]

The term 'electric combi boiler' does not mean instantaneous water heating, just using something that looks like a gas boiler to heat a cylinder and rads

 

https://www.electric-heatingcompany.co.uk/electric-boilers/comet-combi/

[Temp]

Perhaps show him the table here...

 

https://nottenergy.com/resources/energy-cost-comparison/

 

 

An electric combi - 20p per kWH.

ASHP - 7.7p per kWH

Mains Gas - 5.1p per kWH

 

Can he afford nearly three or four times the running cost?

 

Edited August 25, 2019 by Temp

[SteamyTea]

What size is this new house.

[ProDave]

The house is a 3 bed semi.  I have no idea yet who is building the other half, I assume they are being built as a pair.  I am being drip fed information on his project. About the only thing I know for certain Is I will be wiring it.

 

It must be on some development, as he mentioned "one of the other" houses has fitted an ASHP and his big objection seems to be they put it right next to the front door.   I tried to tell him that it can go almost anywhere that suits him, but he seems stuck with not wanting it there so he is not going to have one.  Granted your options on wall space are more limited with a semi than detached.

 

Re that cost comparison table.  I pay 14.8p per KWh for electric so I would expect hot water from an electric boiler to cost 14.8p per KWh not 20p?

[PeterW]

9 hours ago, A_L said:

The term 'electric combi boiler' does not mean instantaneous water heating, just using something that looks like a gas boiler to heat a cylinder and rads

 

https://www.electric-heatingcompany.co.uk/electric-boilers/comet-combi/

 

So basically it’s a cylinder in a steel box with a Willis type system in the top to heat water at a slightly faster rate for DHW... so the “combi” bit is about a combination of DHW and heating in t the same box. Any pricing ..?

[ProDave]

The term "combi" will confuse most customers (as it did me) because we have been used to the term used to describe gas boilers that heat the DHW in real time with no hot water storage tank.

 

Clearly that particular electric "combi" boiler does not do that.

 

When I suggested a "storage boiler" I was thinking of one of the ones based around a big thermal store cylinder that have typically three 3KW immersion heaters and can heat a decent volume of water and use that both for central heating and DHW just like any other thermal store.  The storage volume enables them to work from off peak electricity and mostly only heat up at the off peak times.  They work best with E10.

 

I know only 2 people with electric boilers both say they are expensive to run.  One removed it and replaced it with an oil fired boiler and noticed a reduction in heating costs. Enough said.

[joth]

Surprising there's a whole housing development without any three phase supply nearby.

Regardless, if they're all in this same state, it would seem the ideal situation to invest in a shared ground source loop.

[newhome]

I have electric heating and have a special tariff for heating of 8.8p. It’s still more than gas but certainly not 20p. 

 

[the_r_sole]

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Edited September 26, 2019 by the_r_sole

[Jeremy Harris]

Worth looking at just how much electrical energy a house needs.  Heating is now a relatively low demand, with better insulation and airtightness, so hot water usage seems to dominate.  A shower typically uses around 2.5 to 3 kWh.  Hot water for washing hands, dishwashing, etc might use another 1 kWh per person.  A household of four might use around 15 kWh worth of hot water over 24 hours, so an average power of around 625 W.  Clearly storage capacity might be limited, but it's not hard to store 10 to 15 kWh worth of hot water, or its equivalent as heat stored some other way, like with a Sunamp.

 

It's hard to see why anyone would ever need a high power electric water heating system.  A standard 3 kW immersion heater will only take maybe 5 hours to heat up the hot tank, so could easily do this during the cheap off peak period.

 

I've wondered whether using a cheap ASHP to provide the bulk of the heat into a heat storage system, say up to about 40°C to 45°C, and then having a direct electric heating element that boosts the temperature up to around 55°C to 60°C might not be a pretty good compromise.  Over 50% of the hot water would be provided at a cost of well under 3p/kWh, with the remainder costing around 8p/kWh.  I reckon it should be possible to have loads of electrically heated water for not much more than the cost of heating hot water from mains gas.

[PeterW]

1 hour ago, JSHarris said:

I've wondered whether using a cheap ASHP to provide the bulk of the heat into a heat storage system, say up to about 40°C to 45°C, and then having a direct electric heating element that boosts the temperature up to around 55°C to 60°C might not be a pretty good compromise.  Over 50% of the hot water would be provided at a cost of well under 3p/kWh, with the remainder costing around 8p/kWh.  I reckon it should be possible to have loads of electrically heated water for not much more than the cost of heating hot water from mains gas.

 

That is exactly what I have done - 300 litre UVC on a timed 9Kw ASHP load from 1-3:30am taking it to 45c, then the ASHP kicks over to the UFH slab. Bottom immersion kicks in at 5-6:30am to take the tank to 63c. Towel rails and bathroom UFH mats on a separate circuit to heat up for an hour up to 7am. All main heat loads on E7. 

 

Top immersion is on a switch as a boost if needed (will eventually be Sonoff/ITTTT controlled) so that’s the only heat out of E7 hours. 

Edited August 26, 2019 by PeterW
ASHP size added

[joe90]

1 hour ago, JSHarris said:

I've wondered whether using a cheap ASHP to provide the bulk of the heat into a heat storage system, say up to about 40°C to 45°C, and then having a direct electric heating element that boosts the temperature up to around 55°C to 60°C

 

@ProDave and I heat our DHW to 47 -48’ using the ASHP and don’t find the need to top it up as we have found this temp very adequate fir our needs.

[Jeremy Harris]

9 minutes ago, joe90 said:

 

@ProDave and I heat our DHW to 47 -48’ using the ASHP and don’t find the need to top it up as we have found this temp very adequate fir our needs.

 

 

I guess this works fine for a relatively modest hot water demand, but the tank size does need to be increased a bit to allow for the lower temperature (less cold gets mixed with the hot for things like showers and baths).  For a house with a high hot water demand, there's some merit in increasing the temperature, so as to reduce the size of the hot tank that's needed.

[redtop]

2 hours ago, the_r_sole said:

I did a house (for an electrician) which was fully electric heating - on the older version of sap too. But it was really painful to get through the sap, he would have been a lot cheaper to put an air source in rather than pay for all the extra insulation it needed to make work in the calc! 

I don't think you could get through the new sap without renewables

we have got an A rating on SAP without renewables using a standard gas combi boiler, going for an ASHP reduced the SAP for some reason!

[Jeremy Harris]

20 minutes ago, redtop said:

we have got an A rating on SAP without renewables using a standard gas combi boiler, going for an ASHP reduced the SAP for some reason!

 

 

It would, as SAP weights electricity heavily (and unfairly, IMHO) based on the state of UK power generation when most of it came from burning stuff like coal and oil.  Now much of our electricity generation is from gas and renewables* SAP needs to change, so that it better reflects reality.

 

* As I type this, this is the split of fuels being used to generate electricity in the UK:

 

[A_L]

5 hours ago, PeterW said:

Any pricing ..?

 

This is my preferred link, couldn't find it before!

 

http://www.thermaflowheating.co.uk/single-phase-models/

[ProDave]

4 hours ago, JSHarris said:

...A shower typically uses around 2.5 to 3 kWh.  Hot water for washing hands, dishwashing, etc might use another 1 kWh per person.  A household of four might use around 15 kWh worth of hot water over 24 hours, so an average power of around 625 W. 

Our familly of 3. At the moment the ASHP is clocking up about 15KWh per week heating hot water.  If we guess a COP of 3 (optomistic) that's 45KWh of hot water per week, or 6.42KWh per day.

 

The solar PV will be dumping perhaps another 3KWh per day into the hot water tank so we are using no more than 10KWh of hot water per day.  So I think we are doing quite well in that respect.

[ProDave]

4 hours ago, JSHarris said:

I've wondered whether using a cheap ASHP to provide the bulk of the heat into a heat storage system, say up to about 40°C to 45°C, and then having a direct electric heating element that boosts the temperature up to around 55°C to 60°C might not be a pretty good compromise.  Over 50% of the hot water would be provided at a cost of well under 3p/kWh, with the remainder costing around 8p/kWh.  I reckon it should be possible to have loads of electrically heated water for not much more than the cost of heating hot water from mains gas.

I have pondered that. Indeed our ASHP has that as a built in function, to "stage 1" heat the DHW with the ASHP and then "stage 2" heat it further with it's built in willis heater.

 

I can see that working if you heat your tank once a day and it is starting from reasonably cold.  But to do so requires a tank that will adequately supply all your needs at any time of day.

 

In the real world usage, showers in this house are taken at different times of day and they are by far the biggest user of HW.  So the DHW heating system needs to be "on" all day, ready to re heat the tank when it goes below the target.  So the problem I see is whenever it drops below the "stage 2" target, the willis heater would come on and reheat the tank, meaning the ASHP would not be doing much of the DHW heating.

 

What is working well for us at the moment is just heating DHW to 48 degrees with the ASHP.  During the day the solar PV will top it up to a higher temperature.  The ASHP remains "on" for DHW heating until 10PM.  Nobody usually showers that late so it goes off for the night (part of my strategy for silence at night)

 

There is usually enough heat in the tank for a morning shower, if not the Steible Eltron heater will top it up.  The ASHP does not turn "on" for DHW heating until late morning when on a sunny day there will be enough PV generation to power it.

[Jeremy Harris]

I suspect there is a fairly wide variation in the pattern of use between different households, even ones that outwardly seem similar.  We have friends who never shower, but take baths in the evening, so almost all their hot water use is then, whereas we both shower early in the morning.  For us, using E7 to heat the water in winter works fine, as there's enough left over after the morning showers for hand washing etc through the day.  We're fortunate in that our pattern of use fits well with both summer and winter energy availability, as during the winter the E7 will have heated the water a few hours before use, leaving spare capacity for any boost it might get from excess PV generation, and in summer the hot water is pretty much always recharged completely by lunchtime, and stays hot, so doesn't use much, if any, grid electricity during the overnight off-peak boost.

[AliG]

I don't know about the SAP but thinking about practicality.

 

My landlord in London replaced the gas boiler with an electric "combi". The gas boiler was in an interior space and the requirements for flue inspections are now a pain.

 

Heating and hot water for just me in a 500sq ft flat is fine with this.

 

But it only has a small tank to hold preheated water and cannot fill a bath without running out of hot water. So would be no use for a multi person household.

 

The "combis" shown are simply electric boilers and hot water tanks in one. Running one of these for a family will not be cheap.

 

I don't know where those energy prices came from. Gas is around 3p/kWh and electricity around 14p. You can get them a bit cheaper.

 

To heat 150l of water a day by 40c is almost 8kWh. So water heating would cost 24p a day with gas or 112p a day with electricity. So around £90 a year on gas and £410 on electricity. Worse again if it say a 4 person household. An ASHP would cost more than gas to run, assuming a COP in the 3 range, around £150 a year for DHW. So direct electric is £260 more than an ASHP.

 

Heating costs would probably increase by a similar amount versus an ASHP . What I don't know is the heating requirement of the house. If it is quite low then it might be similar to DHW.

 

So you could argue that an ASHP would save around £500 a year vs direct electric but cost a few grand more.

 

I doubt the numbers work out if you are building for yourself, but they might if you are building for tenants or unsuspecting buyers.The SAP score/EPC will at least try to highlight this issue.

 

 

 

 

 

 

 

Edited August 26, 2019 by AliG

[ProDave]

All electric (no gas) makes a great deal of sense for a landlord as it eliminates annual servicing and safety inspection costs.

[Ed Davies]

6 hours ago, JSHarris said:

I've wondered whether using a cheap ASHP to provide the bulk of the heat into a heat storage system, say up to about 40°C to 45°C, and then having a direct electric heating element that boosts the temperature up to around 55°C to 60°C might not be a pretty good compromise. 

 

We've discussed this before. Surely the HP CoP wouldn't go below 1 even at 60°C so, remind me, what is the point of switching to direct electric heating?