Backup gas boiler: outside the airtight envelope? combi?

[joth]

Probably some silly questions, but just getting started with water/heating design so bear with me!

 

In an energy efficient home with a lot of solar PV, would there be a case for installing a system boiler outside of the airtight envelope?

 

We're planning an EnerPHit home with 8kW of PV. Using a thermal store for DHW and heating (UFH downstairs; towel rails upstairs).

In summer all DHW needs will be from the immersion heater, in winter it will just be on coldest and shortest days, and with lots of guests (e.g. Christmas) we expect to need to drop back to the boiler.

 

Possible locations for boiler are in the laundry room (inside envelope, next to the thermal store) or just the other side of the wall in the garage. The garage was not a obvious choice, but now wondering about it as presumably keeping the flue etc entirely outside the envelope is better for airtightness, with just a pipe having to penetrate it. These pipe runs could be very short (straight through the wall).

What I don't know is if a boiler out in the cold will have more maintenance issues, the frost prevention should protect it but might be wasteful for generating heat we don't actually need, and if there are other drawbacks.

So question 1 is, anyone done this? Or, having put the boiler indoors and found significant issues with it ruining airtightness? (Or any tips for avoiding that?)

 

Second, loosely related question: what type of boiler make sense for a system backup? Ideally we'd make it the smallest size viable for the worst-case heating requirements in winter (i.e. should be fairly low) but that might result in a long wait for a hot shower should the cylinder get emptied on Christmas morning. Would anyone consider using a combi boiler for a backup? So the DHW could flip over to be fed direct from combi instead of the thermal store when it is depleted. Downside is more complex plumbing and control system, more maintenance needed for a rarely used combi(?), and more cost. And I think this would really imply boiler inside the envelope. (Obviously in this situation we couldn't happily run all 3 showers from the combi, but I think that's a reasonable compromise).

So, again, anyone done anything like that?

 

Edited March 31, 2019 by joth

[PeterW]

No problem installing in an unheated garage as long as you fit a frost stat in there with it. Consider building a small enclosure in the garage with the boiler in it and insulate that - quick and simple and having a small room that can also be used for temperature sensitive storage or even lockable so you can keep power tools in there. 

[ProDave]

How are you heating the house?  The PV won't give enough for that for a good part of the winter.

 

Frost prevention should not be an issue. For a start it will have inhibitor in the water which contains atifrfeeze.  But freezing inside an attached garage just is not going to happen.  I have an attached garage with the walls insulated to the same standard as the house, but let down by the "insulated" roller garage door.  In spite of that obvious massive heat leak, and the fact there is no heating at all in the garage, the inside temperature has not dropped below 3 degrees even when it went down to -15 one night.

[PeterW]

14 minutes ago, ProDave said:

For a start it will have inhibitor in the water which contains atifrfeeze. 

 

You cannot rely on that - in a normal system there is less than 0.5% antifreeze in the system.

 

Most - if not all - MIs very clearly say that if you install in an unheated space you have to install a frost stat. 

[joth]

Should have mentioned, still in planning, haven't completed full PHPP so don't know the exact heating requirements. We have a fair bit of the SW face of the house is glass so we think we'll do alright for solar gain. (Certainly inspired from reading on here about using the UFH to help circulate that around the ground floor slab)

But yes, if the boiler looks like it will be in use more often than on rarely, putting it inside the envelope makes sense because then the heat put out by the unit itself is captured inside the house. Still curious if it is a leaky box for airtightness.

If we do put it outside the envelope, putting it in its own insulated box sounds a nice idea. The garage will get fairly cold as it's currently detached from the house; we're building an extension up to join us to it, but this will be the highest speced part of the house insulation wise (being the new-built part)

 

 

[ProDave]

2 minutes ago, PeterW said:

 

You cannot rely on that - in a normal system there is less than 0.5% antifreeze in the system.

 

Most - if not all - MIs very clearly say that if you install in an unheated space you have to install a frost stat. 

The cheap antifreeze / inhibitor I bought from Screwfix at 25% mix is rated to -10.  It's what I use to my outside ASHP.

 

Agreed a frost stat is probably a good idea but it will probably never come on.

[Alexphd1]

For the small cost of a frost stat and pipe stat then its probably not worth the hassle not to fit one, but when it's very cold outside the boiler will be firing anyway. 

Don't think I would bother with building a insulated box around it but would have the pipes heavy insulated and any pumps/motorised valves/filters etc in the house. 

[Nickfromwales]

 

On ‎31‎/‎03‎/‎2019 at 20:23, joth said:

In an energy efficient home with a lot of solar PV, would there be a case for installing a system boiler outside of the airtight envelope?

 

We're planning an EnerPHit home with 8kW of PV. Using a thermal store for DHW and heating (UFH downstairs; towel rails upstairs).

In summer all DHW needs will be from the immersion heater, in winter it will just be on coldest and shortest days, and with lots of guests (e.g. Christmas) we expect to need to drop back to the boiler.

 

Possible locations for boiler are in the laundry room (inside envelope, next to the thermal store) or just the other side of the wall in the garage. The garage was not a obvious choice, but now wondering about it as presumably keeping the flue etc entirely outside the envelope is better for airtightness, with just a pipe having to penetrate it. These pipe runs could be very short (straight through the wall).

What I don't know is if a boiler out in the cold will have more maintenance issues, the frost prevention should protect it but might be wasteful for generating heat we don't actually need, and if there are other drawbacks.

So question 1 is, anyone done this? Or, having put the boiler indoors and found significant issues with it ruining airtightness? (Or any tips for avoiding that?)

 

Second, loosely related question: what type of boiler make sense for a system backup? Ideally we'd make it the smallest size viable for the worst-case heating requirements in winter (i.e. should be fairly low) but that might result in a long wait for a hot shower should the cylinder get emptied on Christmas morning. Would anyone consider using a combi boiler for a backup? So the DHW could flip over to be fed direct from combi instead of the thermal store when it is depleted. Downside is more complex plumbing and control system, more maintenance needed for a rarely used combi(?), and more cost. And I think this would really imply boiler inside the envelope. (Obviously in this situation we couldn't happily run all 3 showers from the combi, but I think that's a reasonable compromise).

So, again, anyone done anything like that?

 

 

Unless your DHW demand is super high, I would not even think about using a thermal store. I only use a TS in conjunction with gas / oil for situations where the client asks for 3 - 4 showers to be running simultaneously. With that kind of setup, sized correctly, you can get constant, instant high-flow DHW.

 

The question should be, in a well insulated airtight build, why bother with the boiler at all? Go for E10 and heat with electricity for a cheap and near zero maintenance option

[joth]

@Nickfromwales thanks for the reply. I am no doubt misusing terminology - by thermal store what I  mean a placeholder for "something" I can dump heat/electricity into when it's cheap (coming from the PV, or E7/10 if we decide to go that way in future) and that we can take heat out of for DHW and space heating at other times of day. The exact selection could be SA, UVC, or a full fledged Thermal Store TBD. But the key thing is being able to bank up energy when the sun is shining, ideally enough to cover over a day or two of overcast, not for throughput it delivers. (2 simultaneous showers every day, 3 very occasionally)

Back of envelope, 160m2 house will have 4000kWh annual heating requirement, which worse case (ignoring any PV generation) looks like about £200 p.a. more on E7 vs gas (using 8p vs 3p per kWh). So at today's rates the boiler should pay for itself.  What could throw all this off kilter will be the cost of moving the gas head. If that's too much, it might push us over to just having it disconnected and going fully electric. Saving on ongoing maintenance would be nice too.... but it all makes me fear we'd then be obliged to get into ASHP and that's not something I'm ready for yet. (The noise, mostly).  GSHP would seem ideal, except for the price....

 

Anyway.... regardless of what our specific design ends up, I'm still academically curious of which is "best" place for a boiler, inside or outside the envelope

 

[Nickfromwales]

12 hours ago, joth said:

@Nickfromwales thanks for the reply. I am no doubt misusing terminology - by thermal store what I  mean a placeholder for "something" I can dump heat/electricity into when it's cheap (coming from the PV, or E7/10 if we decide to go that way in future) and that we can take heat out of for DHW and space heating at other times of day. The exact selection could be SA, UVC, or a full fledged Thermal Store TBD. But the key thing is being able to bank up energy when the sun is shining, ideally enough to cover over a day or two of overcast, not for throughput it delivers. (2 simultaneous showers every day, 3 very occasionally)

Back of envelope, 160m2 house will have 4000kWh annual heating requirement, which worse case (ignoring any PV generation) looks like about £200 p.a. more on E7 vs gas (using 8p vs 3p per kWh). So at today's rates the boiler should pay for itself.  What could throw all this off kilter will be the cost of moving the gas head. If that's too much, it might push us over to just having it disconnected and going fully electric. Saving on ongoing maintenance would be nice too.... but it all makes me fear we'd then be obliged to get into ASHP and that's not something I'm ready for yet. (The noise, mostly).  GSHP would seem ideal, except for the price....

 

Anyway.... regardless of what our specific design ends up, I'm still academically curious of which is "best" place for a boiler, inside or outside the envelope

 

Inside is a no-brainer eg within the heated envelope, but you're losing only a few quid per annum to lose that tbh so not a game-changer. In even a crap garage you'll not have any serious frost risk beyond normal pipe insulation etc plus the frost stat. Only killer with it that far out is the delay getting DHW from the cold start of the combi, up to temp, through the pipe runs and to the taps. That will be a serious delay, especially at the basin taps where there is low flow. 

[joth]

11 hours ago, Nickfromwales said:

Inside is a no-brainer eg within the heated envelope, but you're losing only a few quid per annum to lose that tbh so not a game-changer. In even a crap garage you'll not have any serious frost risk beyond normal pipe insulation etc plus the frost stat. Only killer with it that far out is the delay getting DHW from the cold start of the combi, up to temp, through the pipe runs and to the taps. That will be a serious delay, especially at the basin taps where there is low flow. 

 

My counter thought is: if once is primarily aiming for the "certificate" rather than underlying performance of PH/EnerPHit, then there's no "extra points" on offer to be gained by putting the boiler inside the envelope (AFAIK), but that flue certainly adds extra risk for that all important final airtightness test?  (Even if the flue itself is sealed, it's a bigger hole on the envelope to have to deal with, not to mention a bigger thermal bridge)

I'm sure all will become clear when we get into the detail of PHPP anyway.

[Nickfromwales]

Making a hole in the fabric is nothing to worry about, as long as the person making it is good at their job   

Sealing a duct in first, with a slightly larger diameter than the flue, is what I do. Then you can tape from the internal bore of the duct to the airtight layer far more effectively. 

The flue then simply gets pushed through ( with an equal gap all around between it and the duct ) and intumescent foam injected all round to form a mechanical and airtight fix. When cured, cut the foam back half inch either side and finish the seal with CT1 / similar to protect the foam from insect / rodent attack.

I turned @vivienz‘s airtight layer into Swiss cheese to get various services in / out, and after I put that right it got an AT test result of 0.25ach. ?

Its not the size of the dog in the fight, it’s the size of the fight in the dog    

Edited April 9, 2019 by Nickfromwales
typo / clarity

[joth]

@Nickfromwales very helpful, thanks. ?

 

This is probably a silly question, but I take it all  modern boilers have balanced flue that is totally sealed from the room in a 100% airtight way?  So no need to "shop around" for a special make/model or even passive house certified boiler, like folks have to for wood burners? (Searching online didn't bring up anything on this topic, so I assume it's not complicated, but I know our current elderly boiler is open flue and seems it would be a disaster for airtightness)

 

 

[Nickfromwales]

10 hours ago, joth said:

@Nickfromwales very helpful, thanks. ?

 

This is probably a silly question, but I take it all  modern boilers have balanced flue that is totally sealed from the room in a 100% airtight way?  So no need to "shop around" for a special make/model or even passive house certified boiler, like folks have to for wood burners? (Searching online didn't bring up anything on this topic, so I assume it's not complicated, but I know our current elderly boiler is open flue and seems it would be a disaster for airtightness)

 

 

Yes, correct, and the terminology is "room sealed" ( R/S ) type appliance