Plumbing dhw coil into flow

[Trojan]

Any reason you couldn't plumb a hot water tank in series with the radiators? Would make sense to me given you normally want a higher flow temp through the tank than the radiators themselves.

 

Crude diagram below

[marshian]

Water takes the shortest path?

 

what happens to the flow temp in the rads when the tank is fully heated?

 

the Tank needs 60 deg C to get it to a sensible 50 temp

 

DHWP is the process to follow to separate HW demand from CH so you can run two boiler temps 

[sharpener]

33 minutes ago, Trojan said:

Any reason you couldn't plumb a hot water tank in series with the radiators?

 

Given the temp drop across the coil, with an HP you are increasing the flow temp which depresses the CoP.

 

Also there is the question of putting the whole heating flow through the coil and the resulting pressure drop.

 

Your control solution with a 3-port valve would work but you also need a bypass arrangement for when there is no heating demand at all, which could be done but typical modern controllers are not set up for this.

 

An analogy would be those radios and televisions which had all the valve heater windings in series, the valves all drew 100 (or 300) mA with variable voltage drop. It is a whole new system concept and needs designing in the round for it to work, though it did. A constant 6.3 V supply is a lot easier to design with and you do not get high AC potentials on the heaters.

[Trojan]

 

36 minutes ago, marshian said:

Water takes the shortest path?

There's only one path in this scenario.

37 minutes ago, marshian said:

what happens to the flow temp in the rads when the tank is fully heated?

Valve changes back to only doing the rads.

 

18 minutes ago, sharpener said:

 

Given the temp drop across the coil, with an HP you are increasing the flow temp which depresses the CoP.

Yeah, I'm assuming the minimum you would want to be putting through a heating coil though would be around 50C, even a 15C drop across the coil would probably still be useful in the rads no?

 

22 minutes ago, sharpener said:

 

Also there is the question of putting the whole heating flow through the coil and the resulting pressure drop.

 

Your control solution with a 3-port valve would work but you also need a bypass arrangement for when there is no heating demand at all, which could be done but typical modern controllers are not set up for this.

 

An analogy would be those radios and televisions which had all the valve heater windings in series, the valves all drew 100 (or 300) mA with variable voltage drop. It is a whole new system concept and needs designing in the round for it to work, though it did. A constant 6.3 V supply is a lot easier to design with and you do not get high AC potentials on the heaters.

Yeah, I'm not sure how much resistance a cylinder coil would add to it, presumably that could be overcome with a suitably sized pump though. I was also thinking if you monitored the flow temp after the DHW circuit, the heat pump would overshoot automatically and heat the water as a biproduct of making X flow temp for the radiators.

[sharpener]

16 minutes ago, Trojan said:

Yeah, I'm assuming the minimum you would want to be putting through a heating coil though would be around 50C, even a 15C drop across the coil would probably still be useful in the rads no?

 

Yes. This would work with a boiler perhaps. With an HP, as well as the decrease in CoP you have also the consideration that they usually work with a small dT and a high flow rate which is a load profile you do not have with loads in series.

[Nickfromwales]

Unnecessary nonsense tbh.

 

Heat pumps give the required heat to each duty, either space heating or DHW.

 

Why would you want to slow down the reheat of the cylinder?

 

Also, how would the heat pump choose which temp to output and when, as it would think that in DHW mode it was only servicing a cylinder.

 

Abort!

[JamesPa]

5 minutes ago, Nickfromwales said:

Unnecessary nonsense tbh.

... which exists as a discussion point only because the backward UK heating industry didn't adopt weather compensation for boilers, unlike many of our more enlightened cousins in mainland Europe.  As a result our houses are less comfortable and most are consuming 10% more gas than they need to.

 

When will we ever learn?

[JohnMo]

4 hours ago, Trojan said:

Any reason you couldn't plumb a hot water tank in series with the radiators

Another person overthinking - KISS applies. 3 port on either on flow or return. One side normally closed (by spring return) to cylinder, the other to CH.  If you want to do something similar to drawn use a thermal store, designed for heat pump. Not really sure what you achieved by not keeping it simple.

[sharpener]

1 hour ago, JohnMo said:

Another person overthinking - KISS applies. 3 port on either on flow or return.

 

That's a trifle harsh. It's a reasonable thought experiment. And it is quite simple and as drawn only needs one diverter valve - though it needs a second one to bypass the rad circuit esp if there are any zone valves or TRVs.

 

Any arrangement that will do CH, HW, both or off will need two valves because there are two degrees of freedom and four states. The standard 3 port configuration will not do both at the same time - at least not intentionally.

[marshian]

37 minutes ago, sharpener said:

 

That's a trifle harsh. It's a reasonable thought experiment. And it is quite simple and as drawn only needs one diverter valve - though it needs a second one to bypass the rad circuit esp if there are any zone valves or TRVs.

 

Any arrangement that will do CH, HW, both or off will need two valves because there are two degrees of freedom and four states. The standard 3 port configuration will not do both at the same time - at least not intentionally.

 

When I posted comments I hadn't picked up the heat source was ASHP

 

As I understand it the SCOP or COP improves the lower the flow temp is and it's at it's worst when doing HW at elevated temps

 

Running it thro HW tank coil first then rads is going to be poorer COP more of the time surely??

[Trojan]

23 minutes ago, marshian said:

 

When I posted comments I hadn't picked up the heat source was ASHP

 

As I understand it the SCOP or COP improves the lower the flow temp is and it's at it's worst when doing HW at elevated temps

 

Running it thro HW tank coil first then rads is going to be poorer COP more of the time surely??

Well my thinking was it only needs to go through the tank when the tank needs to be topped up, and then can service the rads with the lower output temp at the same time meaning you don't have to choose between heating and hot water.

 

3 hours ago, Nickfromwales said:

Also, how would the heat pump choose which temp to output and when, as it would think that in DHW mode it was only servicing a cylinder.

 

Abort!

Could just set it up as a zone valve aiming for 40 degree heat temp etc, itll lose the heat in the cylinder.

[JohnMo]

Your drawing just needs to be simplified

 

Hot from ASHP goes to central port of a diverter valve. The normally closed port goes to the cylinder top coil connector. The normally open port goes to central heating.

 

The return piping from central heating goes to ASHP. The cylinder return pipe from bottom connector on cylinder coil tees in the central heating return.

 

After tee on the ASHP side you place the expansion vessel and relief valve. This is sometimes within the ASHP itself already.

 

How it works

ASHP senses cylinder temperature, if it needs topping up, the diverter valve is energised by signal from ASHP, a second operating temp becomes active. Cylinder is heated, heating is off for 30 to 50 mins. Once cylinder is hot diverter is deenergised and normal heating returns to a lower set point.

[Nickfromwales]

11 hours ago, Trojan said:

Well my thinking was it only needs to go through the tank when the tank needs to be topped up, and then can service the rads with the lower output temp at the same time meaning you don't have to choose between heating and hot water.

 

Could just set it up as a zone valve aiming for 40 degree heat temp etc, itll lose the heat in the cylinder.

Nope. That’s a terrible idea. The lower temp means the cylinder would be calling for heat from now until the year 3000…..

 

Just don’t do it. Makes zero sense whatsoever.

 

Just a way to make something already wonderfully simple, complicated as feck. 

[Trojan]

1 hour ago, Nickfromwales said:

Nope. That’s a terrible idea. The lower temp means the cylinder would be calling for heat from now until the year 3000…..

 

Just don’t do it. Makes zero sense whatsoever.

 

Just a way to make something already wonderfully simple, complicated as feck. 

I meant you ask the pump to make 40 degrees to the flow pipe feeding the central heating, meaning it'd actually be higher than that because of the heat loss through the cylinder. I think @sharpener is right though and it wouldn't really work, there likely wouldn't be enough of a drop at those temps, or if there was the heat pump would probably need to be oversized.

[Nickfromwales]

3 hours ago, Trojan said:

I meant you ask the pump to make 40 degrees to the flow pipe feeding the central heating, meaning it'd actually be higher than that because of the heat loss through the cylinder. I think @sharpener is right though and it wouldn't really work, there likely wouldn't be enough of a drop at those temps, or if there was the heat pump would probably need to be oversized.

Let’s relegate this to the back shelf…..