"Cottage" implies small.

Fitting a 28 kW boiler where the demand is for less that 8 kW is going to create so many problems.

You seem to think that a more powerful boiler is going to warm it up more quickly. Thats irrelevant because once the system water has heated up in say 20 minutes the heat output will be limited by the radiator sizes.

The solution is to have a proper sized boiler of perhaps 12 kW and to time or remotely switch it on before you get there or on a timeclock if thats a regular day/time.

Using an electric fan heater to quickly warm up the air is the solution to fast warm up.

Tony

"Cottage" implies small.

Fitting a 28 kW boiler where the demand is for less that 8 kW is going to create so many problems.

You seem to think that a more powerful boiler is going to warm it up more quickly. Thats irrelevant because once the system water has heated up in say 20 minutes the heat output will be limited by the radiator sizes.

The solution is to have a proper sized boiler of perhaps 12 kW and to time or remotely switch it on before you get there or on a timeclock if thats a regular day/time.

Using an electric fan heater to quickly warm up the air is the solution to fast warm up.

Tony

I dont want to get into a spat with a pro Tony, but I can't see how an oversized boiler connected to a thermal store is a problem...? A properly sorted store will call for heat when it has a decent amount of water to reheat - do a long, efficient burn, and switch off until called upon again.

The OP has mentioned that 'groups' may descend on the property... if they get there and want to heat the house / have showers etc at the same time, then using a store as a buffer - and a rapid recovery time from an oversized boiler - would seem to make sense to me.

The problem is mostly with the Vaillant 400 series of boilers which are particularly problematic when oversized!

But any oversized boiler will give a problem because of the limited MINIMUM power output, usually about 33% of the maximum. That means when the flow temp is up to the set value the boiler turns off and stays off for its anticycling delay. That will be the situation where the heating demand is less than the boiler minimum power output.

Furthermore MOST thermal stores are still showing specifications using a store temp of 82 C whereas condensing boilers will be about 12% less efficient when delivering that flow because they are not condensing. That only happens to a lesser extent if the boiler power is lower.

All these problem because YOU want to fit an oversized boiler! Not even for any valid reason!

Tony

Yeah, see your point. In my case I'd have not wanted most of those pre-fitted as it wouldn't have suited my space/packaging requirements.

One downside to the Gledhill stores is that I don't think you get to make custom design options - and you only get one main thermostat tapping. There are pockets for sensors top and bottom for associated controls for an attached wood burner or solar panel - but only one tapping for a stat.

The next one I do will probably be something else - but partly because I'll need more than 0.6 bar pressure capacity (it'll be in the basement with F&E in the loft).

Oops, double post

Furthermore MOST thermal stores are still showing specifications using a store temp of 82 C whereas condensing boilers will be about 12% less efficient when delivering that flow because they are not condensing. That only happens to a lesser extent if the boiler power is lower.

Tony
Oops... Edit

Tony, am I missing something here...

I thought that a condensing boiler was efficient if it is going full pelt?

Mine has a return temp of 55deg set by a TMV on the store, and a max flow temp of about 82deg. I'd assumed it'd be condensing at that?

And I'm not the one wanting to fit an oversized boiler....that is the OP

RWhites

Edit - added 'max' before 'flow temp'

Well if you have a flow temp of 82&#730;C and follow the boiler MIs which normally specify 20&#730;C delta-T - then it follows that the boiler return must be at around 62&#730;C, which is above that at which the boiler will condense. If you throttle the flow down a bit and/or turn the pump down so you have a higher delta-T then you can have condensing and a high output temperature. IMO I can't see why this should be a problem since you won't have a restricted flow like you might have with a rad circuit where most of the rads are throttled down.

But with a thermal store, you don't have the usual problem of oversizing (which happens with any combi in a small property). You never have to work with a reduced flow rate (throttled down rads), and normally shouldn't have an elevated return temp (throttled down rads and bypass going). You only ever take "cool" water from the bottom of the store, and full flow rate, and put it back into the top of the store. So unless your store is hot to the bottom (and 55&#730;C would be quite hot for the bottom of a thermal store) then there's no problem.

Things do change however if you run the boiler indirect - then you have to consider the capacity of the indirect coil and oversizing can be an issue. IMO it would be best if the boiler capacity were less than that of the coil.

Well if you have a flow temp of 82&#730;C and follow the boiler MIs which normally specify 20&#730;C delta-T - then it follows that the boiler return must be at around 62&#730;C, which is above that at which the boiler will condense. If you throttle the flow down a bit and/or turn the pump down so you have a higher delta-T then you can have condensing and a high output temperature. IMO I can't see why this should be a problem since you won't have a restricted flow like you might have with a rad circuit where most of the rads are throttled down.

But with a thermal store, you don't have the usual problem of oversizing (which happens with any combi in a small property). You never have to work with a reduced flow rate (throttled down rads), and normally shouldn't have an elevated return temp (throttled down rads and bypass going). You only ever take "cool" water from the bottom of the store, and full flow rate, and put it back into the top of the store. So unless your store is hot to the bottom (and 55&#730;C would be quite hot for the bottom of a thermal store) then there's no problem.

Things do change however if you run the boiler indirect - then you have to consider the capacity of the indirect coil and oversizing can be an issue. IMO it would be best if the boiler capacity were less than that of the coil.

I've made a small edit to my post, I'd missed a word out.

My return temp is set at 55deg by a TMV - like Simon's I guess - taking cooler water from 2/3rds down the store to mix with a small amount of the hot flow water and returning it to the boiler to (theoretically) maximise efficiency throughout the burn.

In practice, the 'cool return' does eventually hit 65deg right at the end of the cycle, so I'm not sure how the TMV handles that (I'm assuming it allows it through at 65), but in that last half minute of the burn you hear the boiler throttle back before the buffer relay clicks the boiler off.

I'd always thought it was the ideal way to fully utilise a powerful boiler - at maximum efficiency - even if it would be considered 'oversized' on a normal rad circuit without the thermal store.

I would have thought the 400 series is ideally suited to a Thermal Store series, what with its excessively restrictive HEX and apparently quirky initial burn at 80%?

Agree though that oversizing one a boiler is unnecessary.

I am running my thermal store at 70 degrees with 15kW and at half volume and can still draw 25+ litres per minute from it for hot water.

Granted I have decoupled the heating from it for the time being, but The boiler is running that at 58 degrees at the moment.

Return temps never go above 55, and the flue temp only ever goes into the 50's during a large hot water demand.

I'd always thought it was the ideal way to fully utilise a powerful boiler - at maximum efficiency - even if it would be considered 'oversized' on a normal rad circuit without the thermal store.

In many ways it is but you have not defined your meaning of "powerful boiler"

There is no point of having a more powerful boiler than what the heating coil can absorb!

Furthermore, the heat absorbtion is specified with a cold cylinder water temperature. When that rises the absorption reduces. With an overpowered boiler that can cause it to cycle off, reducing efficiency and prolonging reheat time!

You will notice Dan ( as a professional ) chooses to heat his store only to 70 C and uses a boiler rated at less than the maximum absorbtion of the cylinder. That combination creates maximum efficiency.

I will not mention what else Dan does to his boiler though!

Tony

It is almost back to an official factory release... almost.

However, my Excel heat bank is twice the size needed (although I am only using half of it) and can absorb pretty much whatever I can throw at it. There are no coils in mine except a separate 1m2 solar coil at the bottom.

Will be considering playing with adding a second plate in series to see if I can reduce the store temp further.

Ignore all the plastic this was a temporary lash up during the building works. Need to tidy the garage up before i get some newer piccies

Whats in the tupperware?

Looks a bit over complicated!

There is no point of having a more powerful boiler than what the heating coil can absorb!

Furthermore, the heat absorbtion is specified with a cold cylinder water temperature. When that rises the absorption reduces. With an overpowered boiler that can cause it to cycle off, reducing efficiency and prolonging reheat time!

Tony

Tony - There is no heating coil. The 350lt of water sloshing around the store is the very same water pumped through the boiler, so coil absorbtion rates are irrelevant. Personally, and for the very reasons you've stated, I wouldn't want an indirect boiler circuit. (Apart from the solar coil).

That was the sensor altering rig I made up (remember we were discussing the wiring in the CC). Subsequently found a secret parameter in the boilers PCB software that did it all at a touch of a button

Going to be removing it to make space for the condensate pump - getting bored of emptying that bucket twice a day.

Whats in the tupperware?

Looks a bit over complicated!

Isn't the tupperware to catch the drips????

Edit:
Darn it, beat me to it.

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