Solar Panels and Heat Pump fitted but savings not as suggested.

Qyburn

Reed_Richards said:

I take it you have neither, @Qyburn.

I don't, I have an oil boiler. My experience with weather compensation was all with commercial buildings where heating and DHW were on separate pumped circuits so could run at different temperatures simultaneously. WC varied the flow temperature to radiators, not boiler temperatures.

Reed_Richards

It's much the same with a heat pump or a (non-combi) gas boiler.  Both will alternate between heating the DHW cylinder and the heating (radiators/UFH), not both at the same time, with a valve to switch between the two circuits.  Normally the DHW takes priority.  You must have a combi oil boiler, @Qyburn , or your house plumbing would be the same. 

QrizB

Reed_Richards said:

It's much the same with a heat pump or a (non-combi) gas boiler.  Both will alternate between heating the DHW cylinder and the heating (radiators/UFH), not both at the same time, with a valve to switch between the two circuits.  Normally the DHW takes priority.  You must have a combi oil boiler, @Qyburn , or your house plumbing would be the same. 

I can't speak for all systems, but my non-combi boiler system has a three-port mid-position valve (this sort of thing) which can send hot water from the boiler to the radiators, the tank coil or (when in the mid position) to both simultaneously.

I think this is known as Y Plan, and you're describing W Plan. Explanation here.

(Sorry OP, we're getting rather off-topic here.)

Reed_Richards

Yes, three position valves are sometimes used but I have the impression that they're relatively uncommon and/or out of fashion.  They're never used with heat pumps, AFAIK.

Qyburn

Reed_Richards said:

You must have a combi oil boiler, @Qyburn , or your house plumbing would be the same. 

Ours is "S Plan" in Honeywell terminology, separate two port valves for DHW and CH. Both can be open at once, and the boiler/pump come on if either are calling. We don't have WC but if we did then the difference from a heat pump system is that even the lowest boiler temp would be more than hot enough for DHW, which is limited by the cylinder stat.

Reed_Richards

I don't think that "S plan" (or any plumbing scheme where you can heat DHW & CH simultaneously) is compatible with Weather Compensation.  

Qyburn

Reed_Richards said:

I don't think that "S plan" (or any plumbing scheme where you can heat DHW & CH simultaneously) is compatible with Weather Compensation.  

Not with a heat pump or any other system with really low CH flow temp.

But it's fine if the CH temperature is always high enough for DHW. No different to manually changing the boiler's control stat depending on season/weather or expected heating load. We used the office heating system as a test bed and demo for our supervisory controls, and that was how we configured it when we first introduced weather compensation.

Reed_Richards

But if the CH temperature is always high enough for DHW heating then you would be running your boiler pretty inefficiently, possibly not even cold enough to achieve condensing operation, I would have thought.

QrizB

Reed_Richards said:

But if the CH temperature is always high enough for DHW heating then you would be running your boiler pretty inefficiently, possibly not even cold enough to achieve condensing operation, I would have thought.

Well, it works for me.

Most of the year my gas boiler runs a flow temp a little above 50c (based on the input temp at my HW tank). The return temp is usually below 50c, but edges above it once the tank is close to finishing.

This should be plenty cool enough for it to be in condensing mode. There's certainly lots of flow in the condensate pipe!

Only in the depths of winter do I need to edge the flow temp up to 60-65c to get enough heat from the rads to keep the house as warm as Mrs QrizB likes.

I suspect that if we had a proper cold spell with temps in the double-digits negative I might need to turn it up a bit higher.

(I think this is the sort of seasonal weather compensation that Qyburn describes.)

1961Nick

QrizB said:

Reed_Richards said:

But if the CH temperature is always high enough for DHW heating then you would be running your boiler pretty inefficiently, possibly not even cold enough to achieve condensing operation, I would have thought.

Well, it works for me.

Most of the year my gas boiler runs a flow temp a little above 50c (based on the input temp at my HW tank). The return temp is usually below 50c, but edges above it once the tank is close to finishing.

This should be plenty cool enough for it to be in condensing mode. There's certainly lots of flow in the condensate pipe!

Only in the depths of winter do I need to edge the flow temp up to 60-65c to get enough heat from the rads to keep the house as warm as Mrs QrizB likes.

I suspect that if we had a proper cold spell with temps in the double-digits negative I might need to turn it up a bit higher.

(I think this is the sort of seasonal weather compensation that Qyburn describes.)

After a bit of (not totally scientific) testing I've concluded that running a high differential between the cylinder set point & the boiler flow temperature in summer is the most efficient way to operate my boiler. The logic is that the heat loss from the pipework/boiler is reduced due to the shorter cycle time.

Heating a cold cylinder to 50C with a flow temperature of 57C costs 77p whereas increasing the flow temperature to 70C reduces the cost to 51p. The long run of 28mm pipework between the boiler & the cylinder is obviously a major factor.

During winter that long pipe run is eliminated from the calculation because it's shared with the heating & any losses add heat to the house anyway.