Help with new heating and hot water system!

[Harry Bloomfield]

Most people with that set up just leave the DHW on 24/7. A modern cylinder with a big enough coil to run PDHW usually has a heat loss low enough that it doesn't matter

I prefer it not to come on during the night hours, to give the cylinder a quick boost. I have it set to come on at 1500, off at 2000, which normally means we have water hot for baths in the early evening, washing up, and any used hot, is topped up, for next day, plus plenty of hot still available next morning. Coming on at 1500, means the house will be warm, so the ch going off for 30 minutes, will not be noticed.

 

[fixitflav]

Most people with that set up just leave the DHW on 24/7. A modern cylinder with a big enough coil to run PDHW usually has a heat loss low enough that it doesn't matter

That's what I do, and I have W-plan, which has HW priority built-in. I've never found CH having to wait till the HW is satisfied to be a problem, and I'm sure my HW cylinder coil is well scaled-up. With a modern cylinder which can take full boiler output even less of an issue.

 

[drives]

Weather comp and no zoning would be my recommendation. Turning off heating in individual rooms that aren't used in my opinion/experience saves the sum total of naff all, not to mention the faff of keeping doors closed etc. Heat the envelope of the entire building to one temp that you are comfortable with.

 

[bigp26]

In the case of my system, the original installation, back in the 80's, was designed to cope with drafty single glazed windows, and doors, which it coped well with. Since then, I added dg, cavity insulation, and new doors, massively reducing the heat loss. That, made my existing pipes and radiators bigger than needed, so they could achieve the same heating effect, at much lower, more efficient, boiler condensing temperatures.


An heating system designer is now legally required to do a proper heat-loss assessment, before installing a boiler, or an heating system, problem is, many do not, or don't know how. You should insist on this being done, and ask to see the details of the assessment to be sure. You can even do this, yourself, as a back-up. Our original boiler, was a 28Kw, none condensing, none modulating, cast-iron heat exchanger, with a permanent pilot light. Our new one, is an 18Kw Vaillant, 418, which copes perfectly.

Hi - what size / age is your property out of interest?

 

[Harry Bloomfield]

Hi - what size / age is your property out of interest?

It's a 1955 ex-council, 3 bed semi, with a massive garden. Fully refurbed in the mid 80's, rewired, replumbed, 28kw CH added, replastered, repointed. Since when, I have twice replaced the DG, added CWI, demolished walls, added walls and doors, plus a lot more. My present boiler is an 18kw, which is more than adequate.

 

[drives]

We live in a 1800sq ft detached house which isn’t particularly well insulated. Calculated heat loss figures came in around 8kW. 12kW ashp (needed to be slightly higher as the badge rating isn’t quite what it produces as -2 but not far off)- more than adequate to heat the house/supply plenty of DHW.

Why are most system/heat only boilers so massively oversized for what the house actually requires? Our old boiler was 28kW.

 

[Harry Bloomfield]

Why are most system/heat only boilers so massively oversized for what the house actually requires? Our old boiler was 28kW.

Our 28kw, barely coped, with the massive heat loss. We had leaky timber SG frames, drafty timber doors, no CWI, and during the winter, we would still see ice forming on the inside of the windows. The place now, retains any heat put into it, fairly well. The night set back is set for 14C, the boiler has never needed to fire at that, during the night.

 

[drives]

Our 28kw, barely coped, with the massive heat loss. We had leaky timber SG frames, drafty timber doors, no CWI, and during the winter, we would still see ice forming on the inside of the windows. The place now, retains any heat put into it, fairly well. The night set back is set for 14C, the boiler has never needed to fire at that, during the night.

Ok but must be other factors at play there and I guess linked to how gas boilers are typically used with set backs etc. If you had it running 24/7 just inputting heat at the same rate that the building lost it, it wouldn't be 28kW unless your house was really open to the elements surely? I think a 3 bed semi would ordinarily have a heat loss of around 5kW, if that. We've heated our house just fine when I've had big holes in our subfloor as I'm renovating, leaking heat + cold drafts coming in.

 

[fixitflav]

Why are most system/heat only boilers so massively oversized for what the house actually requires? Our old boiler was 28kW.

I didn't know they were, but if so it's likely to be the fault of the installer.

 

[Harry Bloomfield]

If you had it running 24/7 just inputting heat at the same rate that the building lost it, it wouldn't be 28kW unless your house was really open to the elements surely? I think a 3 bed semi would ordinarily have a heat loss of around 5kW, if that.

9 large radiators, and they would get too hot to touch, back then. Now they only need to get barely warm, and boiler can modulate down to 5kw. We have it set to 18C during the day, set back to 14C at night, it only rarely needs to fire. On an evening, we usually take the stat, in the living room, and light the gas fire in there. on around the 1/2 kw setting.

 

[Mister Banks]

I didn't know they were, but if so it's likely to be the fault of the installer.

The average UK home needs 6-8kW so yes the vast majority of system and heatonly boilers in the UK are massively oversized.