Design Temperatures and Radiator Sizing

[sunshine_rag]

Hello Warm People,

I need your help in decoding this set of design temperatures provided by our heat pump installer (installed - end of Jan). we are finding that our kitchens are running way colder. We didn’t fully understand nor were we explained what these design temperatures mean compared to our older boiler setup.

The issue is we are finding our kitchen unusually cold (measuring temperatures at 16-17). Thermostat in hallway set to 18-19. Installer is claiming this is by design and we need to run it as the highest designed temperature (21-22) to get the kitchen to 18.

The kitchen radiators were not replaced as part of our install as they were deemed adequate. They certainly don’t seem to be and this is causing significant discomfort. The kitchen is adjacent to living room and we find it to be a cold pocket. Question is how do I calculate the radiator requirements if I want the kitchen to be at 20?

[arc12 (Adam Cooper)]

Sizing doesn’t look to be a problem.
Have you checked the radiator does not need bleeding. Air in the system is to be expected for a new installation, and can take a while to settle.
Also check that the radiators are balanced. You’ll find lots of info on the web and can go to town with a thermal camera etc but a quick test is to feel the two pipes and compare to other radiators (after the system has got up to temp). An imbalanced system will mean some rooms get more than their fair share of the heat but could also mean the heat pump is getting return water which is too warm, which may harm efficiency. A temp drop of from 5C to 8C across all radiators is a usual aim.

(afterthought - the sizing may have failed to take account of air changes. e.g. is there an extractor fan which is not properly excluding drafts. Are things worse on windy days?)

[Jonathan (Jonathan Frye)]

I’ll have a go at helping you decipher the radiator schedule.

Let me describe what the important columns are:

Design Temperature: This is the room temperature that the designer has decided to target, 18C for the kitchen

Room Heat Loss: This is the rate at which heat is lost (in watts) from the room if it is at the design temperature (18C for the kitchen) and Outside (-2.3C). A higher number means more heat is lost.

Output at Design: This is the amount of heat (in watts) that the radiator can produce, at the design conditions above. A higher number means more heat is produced.

Final Room Output: This is the amount of heat (in watts) that will be produced once the new radiators have been incorporated. A higher number means more heat is produced.

Coverage: This is the percentage of the heat requirement for the room that the radiator can fulfil. For the kitchen this is 190%. 100 means that the radiator would be just enough to heat the room, less than 100 means the radiator wouldn’t be able to heat the room to design temperature. Over 100 means it will be able to exceed the design temperature. A higher number here is not necessarily better but it does mean the radiator is able to produce more heat relative to the room size.

So, your kitchen design temperature has been selected as 18C. The existing radiator should (according to the radiator schedule) be able to meet 190% of the heat loss, i.e. it has loads of spare capacity. The exact calculation for 20C in the kitchen is a little involved but it means the heat loss would be roughly 10% higher, your radiator would still have around 170% coverage, so still easily able to hold that temperature. Actually, according to the schedule you could be knocking into the high 20s with the current radiator.

So, this isn’t matching reality at all. And usually these heat loss estimates are pessimistic, i.e. the radiators are better able to heat the rooms than the schedule suggests.

Your real-world experience is so far out from the heat loss estimate I wonder whether a mistake has been made. The kitchen radiator is shown as the one with the largest coverage in the house, so should be the warmest room.

The kitchen and living room heat losses are shown as about the same. Are they roughly similar rooms? I would expect them to be based on the heat loss survey.

Does the kitchen have a radiator with two panels, with fins on both panels, that is 1100mm wide and 530mm high?

If both those things are true then the heat loss survey is probably correct and the issue is with the radiator itself / plumbing.

Things to check on the radiator is does it feel a similar temperature to other ones in the house (living room particularly)? Is the heat fairly even top to bottom and left to right. You’d expect it to be a bit hotter at the top. Feel the pipes going to the radiator, is one a bit hotter than the other and do they feel similar in temperature to the living room ones?

[sunshine_rag]

Hi Adam,

Thank you for your reply. we had someone over with a thermal camera and they said there was no air in the system or bleeding required for this radiator. As I remember he said he sees a 2degree drop in temperatures between entry/exit. This looks wrong now, I will check again.

is there an extractor fan which is not properly excluding drafts

There is one fan over the hobs, is this what you are referring to? There are no other fans.

We have a large glass sliding door for exit to the garden. I believe they are double glazed.

[sunshine_rag]

Hi Jonathan,

Thank you for taking your time with a very detailed reply explaining all the items in the list. It is very helpful. I am still processing this all. I am surprised (or not) to know that the radiator should be sufficient. This radiator is the largest in the whole house. I will get some pictures later.
Meanwhile I managed to find these (does that help).

[Johntheo (John Carroll)]

What flow/return temperatures are you running at?
If the heat loss is 469 watts and the T50 rated output is 1768 watts then if the flowrate is kept the same you require flow/return temps of 37.3/34.7C/dT 2.6C (with 18C room temp) to emit that 469 watts, 38.5C/33.5C is required at a dT of 5C and 40.0C/32.0C is required at a dT of 8.0C.

[sunshine_rag]

Good question,
The weather compensation curve was set to (-7, 50) and (20, 35) which gives a flow temperature range of 46.1°C to 40.6°C for outdoor temperature range of [0-10]. Which should be more than enough as per your suggestions. The deltaT has been set to 5. Whether these temperatures reach the radiator and deltaT maintained is to be confirmed.

Can I know how did you compute these flow/return values given our radiator specs? Where does 1768 watts come from?

[sunshine_rag]

Also how do I calculate if this radiator is capable of achieving ambient temperature of 20-21 and at what flow temperatures.

[Johntheo (John Carroll)]

Your Kitchen heat loss is stated at 469 watts and the T50 rad output as 1768 watts?.
My calculations are based on a rad output requirement of, 469/1768, 26.52%, see attached.
If you tell me at what outside temp the above based on then I can calculate what rad output is required at say 21C room temp.
Rad Outputs SSR rev 0.xlsx (10.9 KB)

[sunshine_rag]

Ah,Thanks for pointing out the radiator output in that document. It went whoosh by me. The curve/flow temperatures that I had put in for outdoor temperature ranges from 0 to 10. Is 21 room temp achievable when outdoor temp. is in that range?

[Johntheo (John Carroll)]

You can do your own calcs in “calc 2” sheet of the attached spreadsheet but based on my calcs and the supplied data then a rad temp of 38.6C and 469 watts output is required to maintain a kitchen temp of 18C at a OAT of 13.5C, if the OAT fell to say 5C then the required rad temp is 43.3C.
Rad Outputs SSR Rev 1.xlsx (13.0 KB)

Here is a snapshot of those calcs.

[Jonathan (Jonathan Frye)]

Thanks for the additional information, it is some help but I can’t tell you if it is correct because I don’t know anything about your house.

The calculations say that your kitchen room area is 10.5m2, that would be a kitchen that is 3m in one direction and 3.5m in the other direction for example. That’s a plausible size for a kitchen in say a small mid-terrace house. The 3 bedroom end terrace I live in is 17.5m2 for a kitchen/diner.

I have checked my own heat loss calculations for a comparison. This is 3 bedroom end terrace in central Scotland, 30 year old timber framed house, no underfloor insulation, double glazed patio doors like yours. I used an 18C internal temperature and -2.3C external temp, same as yours. I have a design temp heat loss of around 1000w (roughly 500w fabric and 500w ventilation). If I re-calculate for a smaller kitchen, the 10.5m2 yours is, then I get a heat loss of 800w (450w fabric and 350w ventilation).

So your heat loss is calculated as considerably smaller than I calculate were your house the same as mine. Of course yours might not be, it might be much more modern and better insulated. In particularly your ventilation loss of 80w equates to 0.5 air changes / hour, this is very low and would equate to a very recent, well built house.

A rough idea of what your house is (approx size, detached/semi/terrace/flat, age, windows, etc) would be very helpful. Without knowing about the house its difficult to say anything else about what the radiator would be capable of achieving.

[Jonathan (Jonathan Frye)]

I should also have said my kitchen radiator is a K2 1200mm by 600mm, so slightly bigger than yours but not much (to heat a bigger room in a less well insulated house in a colder region). It is able to keep the room warm just fine and I would only exceed 40C flow temperatures at sustained temperatures below -2.3C.

[Johntheo (John Carroll)]

At a OT of -2.3C, the OPs required rad output (by my calcs) is very close to yours, above.

[sunshine_rag]

That is right. We are in a small mid terrace house, about 15 years old. We have a large double glazed sliding patio doors. The dimensions are about right. I think the heat loss calculations are simply run off an algorithm (not measuring anything). I compared our heat loss calculations with neighbours who also had the heat pump installed and the heat loss comes near identical. I am not sure whether this is accurate as well. Also interesting thing to note, from the picture above the radiator (current, at design watts) says it’s only [K2 H530 x L1100] 890 . Is it possible if the heat loss was underestimated this radiator would not able to achieve it given the lower flow temperatures of an heat pump compared to boilers. We never noticed an issue with this radiator with boiler setup.

[sunshine_rag]

Note, we are able to reach 18-19 in the kitchen if we set the thermostat to something like 20-21. But we want to see if we can reach 20-21 in the kitchen/diner with the current rads/heat pump setup. Setting the thermostat any higher seems to have no effect.

[Johntheo (John Carroll)]

Increase the flow temperature.

[sunshine_rag]

Bingo,
I have had to raise the flow temperature (using the curve) significantly to 50 even at higher outdoor temperatures to get it to reasonable temperature ranges (19-20) and I am having to set the thermostat to 21-22 to get these temperatures. which leads me to question,

1. Radiators not balanced - I suspect that the change from the boiler setup to the heat pump setup unbalanced the system as the boiler was in the ground floor and the heat pump tank and outlet is now in the first floor.

2. Radiator type or size - inadequate for our needs.

I am going to have to tweak the balancing myself if the installer don’t show up.

For 2. this is how our radiators look like

How do I know if this type is suitable for a heat pump install which runs at a lower flow temperature compared to boilers?

[John (John Gouk)]

How do you know if they are suitable? Basically, if their heat output at the mean flow temperature is equal to or greater than the heat loss of the space. So if the rad is a K2 or Type 22, like that one you show, and it’s 1200x600mm

• at a mean flow temp of 70C (say 75C in and 65C out) and a room temp of 20C i.e. a difference rad-room temps of 50C, its output will be ~2000W - see the shot from the Stelrad radiator specs, bottom row
  

  

• at a mean flow temp of 35C (say 37.5C in, 32.5C out), room temp 20C for a difference rad-room temps of 15C, its output will be about 430W

The relationship between mean flow temp and output watts is not linear. If you call (RadTemp - RoomTemp) = DT, then Correction Factor = (DTnew/DTrated)^1.3, or in Excel formula-speak for the example above, POWER((15/50),1.3) or 0.209.

So, are your radiators big enough/suitable? Depends on actual flow temp, heat loss of room, target room temp and outside temperature. I have K2s mainly, with a couple of K3s in a larger room with bigger windows, and it’s plenty warm enough, usually 21C.

[Jonathan (Jonathan Frye)]

The radiator is a double panel with fins on both panels, commonly referred to as a K2 radiator. This is what the radiator schedule says you have.

That is suitable for a heat pump and from everything you have said the radiator you have should be more than adequate.

Thanks also for the information about your kitchen. The good news is that the heat loss sounds as if it has been done correctly and your radiator should be more than adequate. So the good news is that there are no fundamental errors in the design and you have a setup that should be capable of easily heating your kitchen.

The bad news of course is that it isn’t.

Radiator balancing - this could be where your problem is. Radiator balancing has nothing to do with where the tank or heat pump is located. Its all to do with how much water is allowed to flow through each radiator compared to the others. If we go back to your original radiator schedule to the Coverage column. This shows that some radiators (like the bedrooms) are around 100% coverage, i.e. they will put out just about the right amount of heat to match the heat loss of the room at design conditions. Other ones (like the kitchen and the hall) show about 190% coverage, i.e. they put out nearly double the required heat than is being lost. All things being equal the hall, kitchen, WC and to some extent Living Room should heat up more than the bedrooms.

What I wonder is whether the installers attempted to balance the kitchen radiator to reduce the heat, and have overdone it (to be fair its not a straightforward task to get right at first attempt). What this means is they will have restricted the amount of water that can flow through the kitchen radiator. If you look at the radiator there should be a valve either side of the radiator, between the pipe that comes up from the floor and the radiator itself. These should have some form of adjustment, it might be a knob or you might remove a cap or something and then turn it will an allen key.

See whether you can increase the flow by adjusting these (anti-clockwise rotation). You’ll then need to leave the heating running for a few hours and see if things have improved. I’d reduce the flow temperature (using the heating curve) back to what it was before doing this.

If its not clear what I mean, can you take a photo of the connection between the radiator and pipe on both sides.